refactor(theme): centralize status colors and overlay tints in CSS vars

Introduce semantic theme variables (e.g. --color-success, --color-danger,
--color-warning, --color-info) and a set of overlay/tint/shadow variables
(--app-hover-tint, --app-raise-tint, --app-soft-tint, --app-overlay-shadow,
--app-menu-shadow, --app-float-shadow, etc.) and use them throughout
app.css in place of many hard-coded color, background and shadow values.

This change:
- replaces literal color tokens used for badges, pills, buttons, menus,
  toasts, borders and file icons with the new semantic variables
- switches several box-shadow and overlay usages to the new shadow vars
- harmonizes light-theme surface, border and scrollbar values to explicit
  variables for easier maintenance

No structural or behavioral changes; this is purely a visual/theming
refactor to make future theme adjustments and dark/light parity simpler.

src/lib/RepoTabs.svelte

@@ -138,7 +138,7 @@
   /* Animate the preview only. On drop, the new DOM order replaces the
      transforms in the same render, so resetting them must not animate. */
   .reordering .repository-tab{transition:transform 160ms ease}
-  .reordering .repository-tab.dragging{z-index:2;transition:none;background:var(--color-surface-raised);border-color:var(--color-accent);box-shadow:0 2px 12px #0005}
+  .reordering .repository-tab.dragging{z-index:2;transition:none;background:var(--color-surface-raised);border-color:var(--color-accent);box-shadow:var(--app-menu-shadow)}
   .repository-navigation.reordering,.reordering .repository-select{cursor:grabbing}
   .repository-select{touch-action:pan-y;user-select:none}
   @media(prefers-reduced-motion:reduce){.reordering .repository-tab{transition:none}}