feat(calls): selectable incoming-call ringtone (#4)

Adds a ringtoneId setting (classic | chime | soft | retro | none) so the
incoming-call ring is no longer hardcoded to call.ogg. The three synth
styles are generated in-browser via a new utils/ringtones.ts module
(mirroring the existing callSounds.ts WebAudio pattern), so no new binary
assets are bundled; 'classic' keeps the existing call.ogg clip and 'none'
is a silent, visual-only incoming-call UI.

- ringtones.ts: startRingtone() loops until stopped; previewRingtone()
  plays a single non-looping preview and auto-cancels the prior preview.
- IncomingCall: ring driven by the setting; <audio> element removed.
- Settings > Calls: Ringtone selector with on-select preview, beside the
  existing Ringtone Volume slider.
- settings.ts: persisted value whitelisted back to a known id.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/settings.ts

@@ -20,6 +20,10 @@ export type NoiseSuppressionMode = 'off' | 'browser' | 'ml';
 // CDN. Its wasm is single-threaded, so no COOP/COEP cross-origin isolation is
 // required (see LOTUS_DENOISE_ENGINEERING_REVIEW.md).
 export type DenoiseModelId = 'rnnoise' | 'speex' | 'dtln' | 'deepfilternet';
+// Incoming-call ringtone. 'classic' is the bundled call.ogg clip; 'chime' /
+// 'soft' / 'retro' are synthesized in-browser (see utils/ringtones.ts);
+// 'none' is silent (visual-only incoming-call UI).
+export type RingtoneId = 'classic' | 'chime' | 'soft' | 'retro' | 'none';
 export type ChatBackground =
   | 'none'
   | 'blueprint'
@@ -148,6 +152,7 @@ export interface Settings {
   afkTimeoutMinutes: number;
 
   callJoinLeaveSound: 'off' | 'chime' | 'soft' | 'retro';
+  ringtoneId: RingtoneId;
   ringtoneVolume: number; // 0–100
 
   seasonalThemeOverride:
@@ -243,6 +248,7 @@ const defaultSettings: Settings = {
   afkTimeoutMinutes: 10,
 
   callJoinLeaveSound: 'chime',
+  ringtoneId: 'classic',
   ringtoneVolume: 70,
 
   seasonalThemeOverride: 'auto',
@@ -273,6 +279,15 @@ export const getSettings = (): Settings => {
         saved.callDenoiseModel === 'deepfilternet'
           ? saved.callDenoiseModel
           : defaultSettings.callDenoiseModel,
+      // Coerce any unknown persisted ringtone id back to the default.
+      ringtoneId:
+        saved.ringtoneId === 'classic' ||
+        saved.ringtoneId === 'chime' ||
+        saved.ringtoneId === 'soft' ||
+        saved.ringtoneId === 'retro' ||
+        saved.ringtoneId === 'none'
+          ? saved.ringtoneId
+          : defaultSettings.ringtoneId,
       composerToolbarButtons: {
         ...DEFAULT_COMPOSER_TOOLBAR,
         ...(saved.composerToolbarButtons ?? {}),