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/components/HelpOverlay.svelte

@@ -2117,7 +2117,7 @@
   .help-search input { width: 100%; height: 40px; padding: 0 68px 0 39px; border-color: var(--color-border-input); border-radius: 8px; background: var(--app-input-bg); color: var(--color-ink); font-size: 12.5px; }
   .help-search kbd { position: absolute; right: 8px; }
   .help-close { justify-self: end; width: 36px; min-height: 36px; padding: 0; border-color: transparent; border-radius: 7px; background: transparent; color: var(--color-ink-muted); transition: color 120ms ease, border-color 120ms ease, background 120ms ease, box-shadow 120ms ease; }
-  .help-close:hover:not(:disabled), .help-close:focus-visible:not(:disabled) { color: #fff; border-color: #f0646d; background: #d93641; box-shadow: inset 0 0 0 1px rgba(255,255,255,.08); }
+  .help-close:hover:not(:disabled), .help-close:focus-visible:not(:disabled) { color: var(--color-on-status); border-color: var(--color-danger); background: var(--color-danger); box-shadow: inset 0 0 0 1px rgba(255,255,255,.08); }
 
   .help-layout { display: grid; grid-template-columns: 250px minmax(0, 1fr); min-height: 0; }
   .help-nav { display: flex; flex-direction: column; min-height: 0; padding: 14px 10px 12px; border-right: 1px solid var(--color-border); background: var(--color-surface-dim); }
@@ -2128,7 +2128,7 @@
   .help-nav-icon { display: grid; place-items: center; color: currentColor; }
   .help-nav-chevron { display: grid; color: var(--color-ink-faint); }
   .help-nav-tip { display: flex; align-items: flex-start; gap: 8px; margin: auto 6px 0; padding: 11px; border-top: 1px solid var(--color-border-subtle); color: var(--color-ink-faint); font-size: 10.5px; line-height: 1.5; }
-  .help-tip-icon { flex: 0 0 auto; display: grid; margin-top: 2px; color: #d8a13d; }
+  .help-tip-icon { flex: 0 0 auto; display: grid; margin-top: 2px; color: var(--color-warning); }
   .help-nav-tip code { color: var(--color-ink-muted); font-family: var(--font-mono); }
 
   .help-content { min-width: 0; min-height: 0; padding: 0 34px 48px; overflow: auto; outline: none; scroll-behavior: smooth; }
@@ -2162,7 +2162,7 @@
   .help-note { display: flex; align-items: flex-start; gap: 9px; margin-top: 15px; padding: 11px 12px; border: 1px solid rgba(90, 140, 248, 0.2); border-radius: 7px; background: rgba(90, 140, 248, 0.07); color: var(--color-ink-muted); font-size: 11px; line-height: 1.5; }
   .help-note-icon { flex: 0 0 auto; display: grid; margin-top: 1px; color: var(--color-accent); }
   .help-note.warning { border-color: rgba(224, 160, 64, 0.24); background: rgba(224, 160, 64, 0.07); }
-  .help-note.warning .help-note-icon { color: #d8a13d; }
+  .help-note.warning .help-note-icon { color: var(--color-warning); }
 
   .help-empty { display: grid; justify-items: center; max-width: 520px; margin: 90px auto 0; text-align: center; }
   .help-empty-icon { display: grid; color: var(--color-ink-faint); }