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/GitLfsDialog.svelte

@@ -209,7 +209,7 @@
 </div>
 
 <style>
-  .lfs-dialog { --lfs-success: #4eca76; --lfs-warning: #f0b648; --lfs-danger: #e86060; width: min(880px, calc(100vw - 32px)); height: auto; max-height: min(660px, calc(100vh - 32px)); display: grid; grid-template-rows: auto minmax(0, 1fr) auto; overflow: hidden; border-radius: 12px; }
+  .lfs-dialog { --lfs-success: var(--color-success); --lfs-warning: var(--color-warning); --lfs-danger: var(--color-danger); width: min(880px, calc(100vw - 32px)); height: auto; max-height: min(660px, calc(100vh - 32px)); display: grid; grid-template-rows: auto minmax(0, 1fr) auto; overflow: hidden; border-radius: 12px; }
   .lfs-dialog-header, .lfs-heading, .dialog-header-actions, .lfs-footer, .lfs-footer > div { display: flex; align-items: center; }
   .lfs-dialog-header { min-height: 58px; justify-content: space-between; padding: 10px 13px; }
   .lfs-heading { gap: 9px; }