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-tauri/tauri.conf.json

@@ -1,8 +1,8 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Psysonic",
-  "version": "1.4.5",
-  "identifier": "dev.psysonic.app",
+  "version": "1.6.0",
+  "identifier": "dev.psysonic.player",
   "build": {
     "beforeDevCommand": "npm run dev",
     "devUrl": "http://localhost:1420",