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/store/downloadModalStore.ts

@@ -0,0 +1,45 @@
+import { create } from 'zustand';
+
+// Module-level callback — not in Zustand state to avoid serialization issues
+let _resolve: ((folder: string | null) => void) | null = null;
+
+interface DownloadModalStore {
+  isOpen: boolean;
+  folder: string;
+  remember: boolean;
+  requestFolder: () => Promise<string | null>;
+  setFolder: (f: string) => void;
+  setRemember: (r: boolean) => void;
+  confirm: (setDownloadFolder: (v: string) => void) => void;
+  cancel: () => void;
+}
+
+export const useDownloadModalStore = create<DownloadModalStore>((set, get) => ({
+  isOpen: false,
+  folder: '',
+  remember: false,
+
+  requestFolder: () =>
+    new Promise<string | null>(resolve => {
+      _resolve = resolve;
+      set({ isOpen: true, folder: '', remember: false });
+    }),
+
+  setFolder: (folder) => set({ folder }),
+  setRemember: (remember) => set({ remember }),
+
+  confirm: (setDownloadFolder) => {
+    const { folder, remember } = get();
+    if (!folder) return;
+    if (remember) setDownloadFolder(folder);
+    _resolve?.(folder);
+    _resolve = null;
+    set({ isOpen: false });
+  },
+
+  cancel: () => {
+    _resolve?.(null);
+    _resolve = null;
+    set({ isOpen: false });
+  },
+}));