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-lock.json

@@ -8,7 +8,7 @@
       "name": "tutorialvault",
       "version": "0.1.0",
       "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",
@@ -411,10 +411,10 @@
         "node": ">=12"
       }
     },
-    "node_modules/@fontsource/fraunces": {
-      "version": "5.2.9",
-      "resolved": "https://registry.npmjs.org/@fontsource/fraunces/-/fraunces-5.2.9.tgz",
-      "integrity": "sha512-XDzuddBtoC7BZgZdBn6b7hsFZY2+V1hgN7yca5fBTKuHjb/lOd45a0Ji8dTUgFhPoL7RdGupo+bC2BFSt6UH8Q==",
+    "node_modules/@fontsource/bricolage-grotesque": {
+      "version": "5.2.10",
+      "resolved": "https://registry.npmjs.org/@fontsource/bricolage-grotesque/-/bricolage-grotesque-5.2.10.tgz",
+      "integrity": "sha512-V2xS+1P7C8IrSypXLUx/bLtX/LsTlYtV2k2CsU+S/0t8qepZ2hvKSlyJIx7Ub/iY8Bbnj+IjAuUF9nvFz+BbIg==",
       "license": "OFL-1.1",
       "funding": {
         "url": "https://github.com/sponsors/ayuhito"