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

package.json

@@ -9,7 +9,7 @@
     "tauri": "tauri"
   },
   "dependencies": {
-    "@fontsource/fraunces": "^5.2.9",
+    "@fontsource/bricolage-grotesque": "^5.2.10",
     "@fontsource/inter": "^5.2.8",
     "@fontsource/space-mono": "^5.2.9",
     "@fortawesome/fontawesome-free": "^7.2.0",