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/ConfirmDialog.svelte

@@ -211,14 +211,14 @@
   }
   .confirm-dialog.danger .unified-dialog-icon {
     border-color: rgba(255, 90, 103, 0.28);
-    color: #ff9aa4;
+    color: var(--color-danger);
     background: rgba(255, 90, 103, 0.09);
   }
   .confirm-dialog .discard-target-list { max-height: 148px; }
   .confirm-dialog .discard-warning-text {
     padding: 9px 10px;
     border-left: 2px solid rgba(255, 90, 103, 0.55);
-    color: #f2aeb5;
+    color: var(--color-danger);
     background: rgba(255, 90, 103, 0.055);
     font-size: 11.5px;
     font-weight: 600;
@@ -257,7 +257,7 @@
     background: color-mix(in srgb, var(--color-accent) 5%, transparent);
   }
   .confirm-dialog:not(.danger) .confirm-check input { accent-color: var(--color-accent); }
-  .confirm-dialog .confirm-check input { width: 15px; height: 15px; margin-top: 1px; accent-color: #e86060; }
+  .confirm-dialog .confirm-check input { width: 15px; height: 15px; margin-top: 1px; accent-color: var(--color-danger); }
   .confirm-dialog .confirm-check span { display: grid; gap: 2px; min-width: 0; }
   .confirm-dialog .confirm-check strong { color: var(--color-ink); font-size: 12.5px; font-weight: 650; }
   .confirm-dialog .confirm-check small { color: var(--color-ink-dim); font-size: 11.5px; }