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/i18n.ts

@@ -123,6 +123,7 @@ const enTranslation = {
     albumCount_one: '{{count}} Album',
     albumCount_other: '{{count}} Albums',
     openedInBrowser: 'Opened in browser',
+    featuredOn: 'Also Featured On',
   },
   favorites: {
     title: 'Favorites',
@@ -241,6 +242,10 @@ const enTranslation = {
     use: 'Use',
     add: 'Add',
     active: 'Active',
+    download: 'Download',
+    chooseDownloadFolder: 'Choose download folder',
+    noFolderSelected: 'No folder selected',
+    rememberDownloadFolder: 'Remember this folder',
   },
   settings: {
     title: 'Settings',
@@ -294,6 +299,7 @@ const enTranslation = {
     downloadsDefault: 'Default Downloads Folder',
     pickFolder: 'Select',
     pickFolderTitle: 'Select Download Folder',
+    clearFolder: 'Clear download folder',
     logout: 'Logout',
     aboutTitle: 'About Psysonic',
     aboutDesc: 'A desktop music player for Subsonic-compatible servers (Navidrome, Gonic, and others). Streams your self-hosted music library with a clean, modern interface styled after the Catppuccin colour palette.',
@@ -304,6 +310,7 @@ const enTranslation = {
     aboutVersion: 'Version',
     aboutBuiltWith: 'Built with Tauri · React · TypeScript · Rust/rodio',
     aboutAiCredit: 'Developed with the support of Claude Code by Anthropic',
+    changelog: 'Changelog',
     randomMixTitle: 'Random Mix',
     randomMixBlacklistTitle: 'Custom Filter Keywords',
     randomMixBlacklistDesc: 'Songs are excluded when any keyword matches their genre, title, or album (active when the checkbox above is on).',
@@ -311,6 +318,22 @@ const enTranslation = {
     randomMixBlacklistAdd: 'Add',
     randomMixBlacklistEmpty: 'No custom keywords added yet.',
     randomMixHardcodedTitle: 'Built-in keywords (active when checkbox is on)',
+    tabPlayback: 'Playback',
+    tabLibrary: 'Library',
+    tabAppearance: 'Appearance',
+    tabServer: 'Server',
+    tabAbout: 'About',
+    playbackTitle: 'Playback',
+    replayGain: 'Replay Gain',
+    replayGainDesc: 'Normalize track volume using ReplayGain metadata',
+    replayGainMode: 'Mode',
+    replayGainTrack: 'Track',
+    replayGainAlbum: 'Album',
+    crossfade: 'Crossfade',
+    crossfadeDesc: 'Fade between tracks',
+    crossfadeSecs: '{{n}}s',
+    gapless: 'Gapless Playback',
+    gaplessDesc: 'Pre-buffer next track to eliminate gaps between songs',
   },
   help: {
     title: 'Help',
@@ -564,6 +587,7 @@ const deTranslation = {
     albumCount_one: '{{count}} Album',
     albumCount_other: '{{count}} Alben',
     openedInBrowser: 'Im Browser geöffnet',
+    featuredOn: 'Auch enthalten auf',
   },
   favorites: {
     title: 'Favoriten',
@@ -682,6 +706,10 @@ const deTranslation = {
     use: 'Verwenden',
     add: 'Hinzufügen',
     active: 'Aktiv',
+    download: 'Herunterladen',
+    chooseDownloadFolder: 'Download-Ordner wählen',
+    noFolderSelected: 'Kein Ordner ausgewählt',
+    rememberDownloadFolder: 'Ordner merken',
   },
   settings: {
     title: 'Einstellungen',
@@ -735,6 +763,7 @@ const deTranslation = {
     downloadsDefault: 'Standard-Downloads-Ordner',
     pickFolder: 'Auswählen',
     pickFolderTitle: 'Download-Ordner auswählen',
+    clearFolder: 'Download-Ordner löschen',
     logout: 'Abmelden',
     aboutTitle: 'Über Psysonic',
     aboutDesc: 'Ein Desktop-Musikplayer für Subsonic-kompatible Server (Navidrome, Gonic u. a.). Streame deine selbst gehostete Musikbibliothek mit einer modernen Oberfläche im Catppuccin-Design.',
@@ -745,6 +774,7 @@ const deTranslation = {
     aboutVersion: 'Version',
     aboutBuiltWith: 'Gebaut mit Tauri · React · TypeScript · Rust/rodio',
     aboutAiCredit: 'Mit freundlicher Unterstützung von Claude Code by Anthropic',
+    changelog: 'Changelog',
     randomMixTitle: 'Zufallsmix',
     randomMixBlacklistTitle: 'Eigene Filter-Keywords',
     randomMixBlacklistDesc: 'Songs werden ausgeschlossen, wenn ein Keyword auf Genre, Titel oder Album zutrifft (aktiv wenn die Checkbox oben an ist).',
@@ -752,6 +782,22 @@ const deTranslation = {
     randomMixBlacklistAdd: 'Hinzufügen',
     randomMixBlacklistEmpty: 'Noch keine eigenen Keywords hinzugefügt.',
     randomMixHardcodedTitle: 'Eingebaute Keywords (aktiv wenn Checkbox an)',
+    tabPlayback: 'Wiedergabe',
+    tabLibrary: 'Bibliothek',
+    tabAppearance: 'Darstellung',
+    tabServer: 'Server',
+    tabAbout: 'Info',
+    playbackTitle: 'Wiedergabe',
+    replayGain: 'Replay Gain',
+    replayGainDesc: 'Lautstärke mit ReplayGain-Metadaten normalisieren',
+    replayGainMode: 'Modus',
+    replayGainTrack: 'Track',
+    replayGainAlbum: 'Album',
+    crossfade: 'Crossfade',
+    crossfadeDesc: 'Überblendung zwischen Tracks',
+    crossfadeSecs: '{{n}}s',
+    gapless: 'Nahtlose Wiedergabe',
+    gaplessDesc: 'Nächsten Track vorpuffern um Lücken zwischen Songs zu vermeiden',
   },
   help: {
     title: 'Hilfe',