feat: Replay Gain, Crossfade, Download Folder Modal, Changelog in Settings (v1.6.0)

### Audio
- Replay Gain support (track + album mode, configurable pre-gain, hard limiter)
- Crossfade between tracks (configurable duration 1–10 s)
- Gapless preloading ⚠️ experimental/alpha — enable in Settings → Playback
- Atomic sink swap: old track plays until new one is decoded, then stops cleanly

### UI / UX
- Settings redesigned with tab navigation (General, Playback, About)
- Changelog viewer in Settings → About with collapsible version entries
- Download Folder Modal: choose folder + "remember" checkbox per-download
- EQ popup now accessible directly from the Player Bar
- "Also Featured On" section on Artist pages for non-album appearances

### Fixes
- Bundle identifier changed from dev.psysonic.app → dev.psysonic.player (fixes macOS warning)
- Version sync: all four version files (package.json, Cargo.toml, tauri.conf.json, PKGBUILD) now at 1.6.0

### Known Issues
- FLAC seeking via waveform seekbar does not work (MP3/OGG unaffected) — under investigation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -29,6 +29,7 @@ import SearchResults from './pages/SearchResults';
 import NowPlayingPage from './pages/NowPlaying';
 import FullscreenPlayer from './components/FullscreenPlayer';
 import ContextMenu from './components/ContextMenu';
+import DownloadFolderModal from './components/DownloadFolderModal';
 import ConnectionIndicator from './components/ConnectionIndicator';
 import OfflineOverlay from './components/OfflineOverlay';
 import { useConnectionStatus } from './hooks/useConnectionStatus';
@@ -189,6 +190,7 @@ function AppShell() {
         <FullscreenPlayer onClose={toggleFullscreen} />
       )}
       <ContextMenu />
+      <DownloadFolderModal />
     </div>
   );
 }