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/components/DownloadFolderModal.tsx

@@ -0,0 +1,56 @@
+import { FolderOpen } from 'lucide-react';
+import { open as openDialog } from '@tauri-apps/plugin-dialog';
+import { useTranslation } from 'react-i18next';
+import { useDownloadModalStore } from '../store/downloadModalStore';
+import { useAuthStore } from '../store/authStore';
+
+export default function DownloadFolderModal() {
+  const { isOpen, folder, remember, setFolder, setRemember, confirm, cancel } = useDownloadModalStore();
+  const setDownloadFolder = useAuthStore(s => s.setDownloadFolder);
+  const { t } = useTranslation();
+
+  const handleBrowse = async () => {
+    const selected = await openDialog({ directory: true, multiple: false, title: t('common.chooseDownloadFolder') });
+    if (selected && typeof selected === 'string') setFolder(selected);
+  };
+
+  if (!isOpen) return null;
+
+  return (
+    <>
+      <div className="eq-popup-backdrop" onClick={cancel} style={{ zIndex: 210 }} />
+      <div className="eq-popup" style={{ zIndex: 211, width: 'min(480px, 92vw)', gap: 0 }}>
+        <div className="eq-popup-header">
+          <span className="eq-popup-title">{t('common.chooseDownloadFolder')}</span>
+        </div>
+
+        <div style={{ padding: '16px 0 12px' }}>
+          <div className="download-folder-pick-row">
+            <span className="download-folder-path">
+              {folder || t('common.noFolderSelected')}
+            </span>
+            <button className="btn btn-ghost" onClick={handleBrowse} style={{ flexShrink: 0 }}>
+              <FolderOpen size={15} /> {t('settings.pickFolder')}
+            </button>
+          </div>
+
+          <label className="download-remember-row">
+            <input type="checkbox" checked={remember} onChange={e => setRemember(e.target.checked)} />
+            <span>{t('common.rememberDownloadFolder')}</span>
+          </label>
+        </div>
+
+        <div className="download-modal-actions">
+          <button className="btn btn-ghost" onClick={cancel}>{t('common.cancel')}</button>
+          <button
+            className="btn btn-primary"
+            onClick={() => confirm(setDownloadFolder)}
+            disabled={!folder}
+          >
+            {t('common.download')}
+          </button>
+        </div>
+      </div>
+    </>
+  );
+}