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/styles/layout.css

@@ -489,6 +489,74 @@
   min-width: 0;
 }
 
+.player-eq-btn {
+  color: var(--text-muted);
+  flex-shrink: 0;
+  transition: color 0.15s;
+}
+.player-eq-btn:hover,
+.player-eq-btn.active {
+  color: var(--accent);
+}
+
+/* ─── EQ Popup ─── */
+.eq-popup-backdrop {
+  position: fixed;
+  inset: 0;
+  z-index: 200;
+  background: rgba(0, 0, 0, 0.35);
+  backdrop-filter: blur(2px);
+}
+
+.eq-popup {
+  position: fixed;
+  z-index: 201;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  width: min(700px, 92vw);
+  background: var(--bg-sidebar);
+  border: 1px solid var(--border);
+  border-radius: 14px;
+  box-shadow: 0 24px 60px rgba(0, 0, 0, 0.5);
+  padding: 20px;
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+}
+.eq-popup .eq-wrap {
+  max-width: none;
+}
+
+.eq-popup-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.eq-popup-title {
+  font-size: 15px;
+  font-weight: 600;
+  color: var(--text-primary);
+}
+
+.eq-popup-close {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  border-radius: 6px;
+  background: var(--bg-hover);
+  border: 1px solid var(--border);
+  color: var(--text-muted);
+  cursor: pointer;
+  transition: background 0.15s, color 0.15s;
+}
+.eq-popup-close:hover {
+  background: var(--accent);
+  color: var(--bg-app);
+}
 
 /* ─── Queue Panel ─── */
 .queue-panel {