feat(waveform): real amplitude waveform via Symphonia + smooth crossfade

Add Rust command `compute_waveform` that downloads the audio file,
seek-samples 500 evenly-spaced frames with Symphonia (fast path) and
computes peak amplitudes. Percentile-based normalisation (p5→p95)
preserves visible dynamics even for heavily compressed music.

Frontend caches results per track (module-level Map), shows the
pseudo-random fallback immediately and crossfades to the real waveform
over 400 ms (ease-in-out) once the computation finishes.

Also: Settings input-tab reset buttons now use btn-danger styling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/Settings.tsx

@@ -1161,7 +1161,7 @@ export default function Settings() {
           </div>
           <div className="settings-card">
             <div style={{ display: 'flex', justifyContent: 'flex-end', marginBottom: '12px' }}>
-              <button className="btn btn-ghost" style={{ fontSize: 12 }} onClick={() => { kb.resetToDefaults(); setListeningFor(null); }}>
+              <button className="btn btn-danger" style={{ fontSize: 12 }} onClick={() => { kb.resetToDefaults(); setListeningFor(null); }}>
                 {t('settings.shortcutsReset')}
               </button>
             </div>
@@ -1247,7 +1247,7 @@ export default function Settings() {
           </p>
           <div className="settings-card">
             <div style={{ display: 'flex', justifyContent: 'flex-end', marginBottom: '12px' }}>
-              <button className="btn btn-ghost" style={{ fontSize: 12 }} onClick={() => { gs.resetAll(); setListeningForGlobal(null); }}>
+              <button className="btn btn-danger" style={{ fontSize: 12 }} onClick={() => { gs.resetAll(); setListeningForGlobal(null); }}>
                 {t('settings.shortcutsReset')}
               </button>
             </div>