fix: WCAG AAA contrast compliance, speed menu z-index, custom app icon

- Fix all text colors to meet WCAG 2.2 AAA 7:1 contrast ratios against
  dark backgrounds (--textMuted, --textDim, hover states across playlist,
  player, panels, tooltips)
- Fix speed menu rendering behind seek bar by correcting z-index stacking
  context (.controls z-index:10, .miniCtl z-index:3, .seek z-index:2)
- Replace default Tauri icons with custom TutorialVault icon across all
  required sizes (32-512px PNGs, ICO, ICNS, Windows Square logos)
- Update README: Fraunces → Bricolage Grotesque font reference
- Add collapsible dock pane persistence and keyboard-adjustable dividers

README.md

@@ -282,7 +282,7 @@ TutorialVault is a [Tauri v2](https://v2.tauri.app/) application with a Rust bac
 - `store.ts` -- Shared state, pure utilities, cross-module callback registry
 - `api.ts` -- Typed wrapper around Tauri's invoke API
 
-**🎨 Design theme.** The interface uses a "Cold Open" dark theme built on cool slate backgrounds (`#0f1117` base) with a steel blue accent (`#88A4C4`). Typography uses [Fraunces](https://fonts.google.com/specimen/Fraunces) for headings, [Inter](https://fonts.google.com/specimen/Inter) for body text, and [Space Mono](https://fonts.google.com/specimen/Space+Mono) for monospace elements. All fonts are bundled -- no external requests at runtime.
+**🎨 Design theme.** The interface uses a "Cold Open" dark theme built on cool slate backgrounds (`#0f1117` base) with a steel blue accent (`#88A4C4`). Typography uses [Bricolage Grotesque](https://fonts.google.com/specimen/Bricolage+Grotesque) for headings, [Inter](https://fonts.google.com/specimen/Inter) for body text, and [Space Mono](https://fonts.google.com/specimen/Space+Mono) for monospace elements. All fonts are bundled -- no external requests at runtime.
 
 **💾 State is local.** All data is stored in JSON files with atomic writes and backup rotation. No database, no server, no network requests beyond the one-time ffmpeg download.