refactor(ui): centralize sync colors into CSS variables

Introduce --color-sync-ahead and --color-sync-behind and update sync
indicators to use those variables. Replace hardcoded sync colors with
var() and color-mix() to unify backgrounds, shadows, and graph accents
across themes, and add light-theme overrides to preserve contrast.

- Add theme-level variables for ahead/behind sync colors
- Replace literal color values with var() and color-mix() usages
- Update dashboard styles to reference the new sync variables

src/lib/components/RepositoryDashboard.svelte

@@ -407,7 +407,7 @@
   .dashboard-header{display:flex;min-height:58px;align-items:center;justify-content:space-between;gap:16px;padding:8px 20px;border-bottom:1px solid var(--color-border-subtle)}.dashboard-header h1{margin:0;font-size:18px;line-height:1.2;font-weight:700}.header-actions,.workspace-tools{display:flex;align-items:center;gap:8px}.header-actions button,.workspace-tools button{display:inline-flex;min-height:30px;align-items:center;gap:6px;padding:0 10px;border:1px solid var(--color-border);background:var(--app-button-bg)}button.primary{border-color:var(--color-primary);background:var(--color-primary);color:#fff}
   .dashboard-toolbar{display:flex;min-height:48px;align-items:center;gap:8px;padding:8px 20px;border-bottom:1px solid var(--color-border-subtle)}.dashboard-search{display:flex;width:300px;height:30px;align-items:center;gap:8px;padding:0 9px;border:1px solid var(--color-border-input);border-radius:var(--ui-radius-sm);color:var(--color-ink-faint);background:var(--app-input-bg)}.dashboard-search:focus-within{border-color:var(--color-accent);box-shadow:0 0 0 2px color-mix(in srgb,var(--color-primary) 14%,transparent)}.dashboard-search :global(svg){flex:0 0 auto}.dashboard-search input{width:100%;min-width:0;height:100%;padding:0;border:0;border-radius:0;outline:0;color:var(--color-ink);background:transparent;box-shadow:none}.dashboard-search input:focus,.dashboard-search input:focus-visible{border:0;outline:0;box-shadow:none}.workspace-tools{margin-left:auto}.workspace-tools :global(.workspace-select){width:240px}.workspace-tools :global(.workspace-select .select-menu-trigger){height:30px;min-height:30px;padding:0 9px;border-color:var(--color-border-input);background:var(--app-input-bg);font-size:12px;font-weight:500}.workspace-tools .workspace-delete{width:30px;min-width:30px;padding:0;border-color:color-mix(in srgb,#df626b 55%,var(--color-border));color:#df747b;background:color-mix(in srgb,#c92f3a 8%,var(--app-button-bg))}.workspace-tools .workspace-delete:hover:not(:disabled){border-color:#df626b;color:#fff;background:#c93b45;box-shadow:0 0 0 2px color-mix(in srgb,#c92f3a 18%,transparent)}
   .dashboard-summary{padding:8px 20px;color:var(--color-ink-muted)}.dashboard-summary span{padding:0 6px;color:var(--color-ink-faint)}.dashboard-content{flex:1;min-height:0;overflow:auto;padding:0 20px 24px}.dashboard-section+.dashboard-section{margin-top:20px}.section-header{min-height:31px;margin-bottom:6px}.section-toggle{display:flex;width:100%;min-height:31px;align-items:center;justify-content:flex-start;gap:7px;padding:0;border:0;background:transparent;color:var(--color-ink);text-align:left}.section-toggle:hover:not(:disabled){border:0;background:transparent;color:var(--color-ink)}.section-toggle :global(svg){flex:0 0 auto;color:var(--color-ink-muted)}.section-toggle strong{font-size:13px;line-height:1.2;font-weight:750}.section-toggle span{color:var(--color-ink-faint);font-weight:400}.section-toggle i{height:1px;flex:1;background:var(--color-border-subtle)}.category-empty{margin:0;padding:9px 12px;border:1px solid var(--color-border-subtle);color:var(--color-ink-faint);background:var(--color-surface)}
-  .repo-grid{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:10px}.repo-card{position:relative;min-width:0;height:138px;border:1px solid var(--color-border);background:var(--color-surface)}.repo-card:hover,.repo-card:focus-within{border-color:var(--color-border-input);background:var(--color-surface-hover)}.card-main{display:flex;width:100%;height:100%;flex-direction:column;align-items:flex-start;gap:7px;padding:12px 104px 11px 14px;border:0;background:transparent;text-align:left}.card-main:focus-visible{outline:2px solid var(--color-accent);outline-offset:-2px}.card-title{display:flex;max-width:100%;align-items:center;gap:8px}.card-title strong{overflow:hidden;font-size:14px;font-weight:700;text-overflow:ellipsis;white-space:nowrap}.card-branch,.card-status,.card-status>span{display:flex;align-items:center;gap:6px}.card-branch{max-width:100%;overflow:hidden;color:var(--color-ink-muted);text-overflow:ellipsis;white-space:nowrap}.card-status{flex-wrap:wrap;gap:8px 18px;margin-top:auto}.card-status :global(.filled){fill:currentColor}.changed,.behind{color:#eeb94e}.clean{color:#68c878}.ahead{color:var(--color-accent)}.muted{color:var(--color-ink-muted)}
+  .repo-grid{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:10px}.repo-card{position:relative;min-width:0;height:138px;border:1px solid var(--color-border);background:var(--color-surface)}.repo-card:hover,.repo-card:focus-within{border-color:var(--color-border-input);background:var(--color-surface-hover)}.card-main{display:flex;width:100%;height:100%;flex-direction:column;align-items:flex-start;gap:7px;padding:12px 104px 11px 14px;border:0;background:transparent;text-align:left}.card-main:focus-visible{outline:2px solid var(--color-accent);outline-offset:-2px}.card-title{display:flex;max-width:100%;align-items:center;gap:8px}.card-title strong{overflow:hidden;font-size:14px;font-weight:700;text-overflow:ellipsis;white-space:nowrap}.card-branch,.card-status,.card-status>span{display:flex;align-items:center;gap:6px}.card-branch{max-width:100%;overflow:hidden;color:var(--color-ink-muted);text-overflow:ellipsis;white-space:nowrap}.card-status{flex-wrap:wrap;gap:8px 18px;margin-top:auto}.card-status :global(.filled){fill:currentColor}.changed{color:#eeb94e}.behind{color:var(--color-sync-behind)}.clean{color:#68c878}.ahead{color:var(--color-sync-ahead)}.muted{color:var(--color-ink-muted)}
   .card-actions{position:absolute;top:8px;right:8px;z-index:2;display:flex;align-items:center;gap:2px}.card-actions button{display:grid;width:26px;height:26px;place-items:center;padding:0;border:0;color:var(--color-ink-muted);background:transparent}.card-actions button:hover:not(:disabled){color:var(--color-ink);background:var(--color-surface-dim)}.card-actions button.active{color:var(--color-accent)}.card-actions button.active :global(svg){fill:color-mix(in srgb,var(--color-accent) 22%,transparent)}.card-actions .pr-badge{display:flex;width:auto;min-width:31px;grid-template-columns:none;align-items:center;justify-content:center;gap:4px;padding:0 5px;color:var(--color-ink-faint);font-size:11px}.card-actions .pr-badge:disabled{opacity:.72}.card-actions .pr-badge.has-open-prs{color:var(--color-accent)}.card-actions .pr-badge.pr-error{color:#e0a35b}.card-actions .pr-badge:hover:not(:disabled){color:var(--color-accent);background:color-mix(in srgb,var(--color-accent) 9%,transparent)}
   .dashboard-footer{display:flex;min-height:30px;align-items:center;padding:0 20px;border-top:1px solid var(--color-border-subtle);color:var(--color-ink-muted)}
   .workspace-dialog{width:min(420px,calc(100vw - 32px));margin:auto;padding:0;border:1px solid var(--color-border);color:var(--color-ink);background:var(--app-dialog-bg);box-shadow:var(--app-dialog-shadow);font-size:12px}.workspace-dialog::backdrop{background:var(--app-dialog-backdrop)}.workspace-dialog form>header,.workspace-dialog form>footer{display:flex;align-items:center;justify-content:space-between;padding:10px 12px;background:var(--app-dialog-chrome)}.workspace-dialog form>header{border-bottom:1px solid var(--color-border)}.workspace-dialog form>footer{justify-content:flex-end;gap:8px;border-top:1px solid var(--color-border)}.workspace-dialog h2{margin:0;font-size:13px}.workspace-dialog header button{width:26px;height:26px;padding:0;border:0;background:transparent}.workspace-dialog form>label,.workspace-dialog form>input,.workspace-dialog fieldset,.dialog-error{margin-right:12px;margin-left:12px}.workspace-dialog form>label{display:block;margin-top:12px;margin-bottom:5px;color:var(--color-ink-muted)}.workspace-dialog form>input{width:calc(100% - 24px);height:30px;padding:0 8px;border:1px solid var(--color-border-input);color:var(--color-ink);background:var(--app-input-bg)}.dialog-error{margin-top:7px;color:#ed9292}.workspace-dialog fieldset{margin-top:14px;margin-bottom:14px;padding:0;border:0}.workspace-dialog legend{margin-bottom:6px;color:var(--color-ink-muted)}.workspace-repos{max-height:190px;overflow:auto;border:1px solid var(--color-border-subtle)}.workspace-repos label{display:grid;grid-template-columns:auto minmax(100px,auto) 1fr;align-items:center;gap:8px;min-height:32px;padding:4px 8px;border-bottom:1px solid var(--color-border-subtle)}.workspace-repos label:last-child{border-bottom:0}.workspace-repos small{overflow:hidden;color:var(--color-ink-faint);text-overflow:ellipsis;white-space:nowrap}.workspace-dialog footer button{display:inline-flex;min-height:28px;align-items:center;gap:6px;padding:0 10px;border:1px solid var(--color-border);background:var(--app-button-bg)}