feat(calls): configurable ringtone volume in Settings (Bug #4 partial)

Adds 'Ringtone Volume' slider (0–100, default 70%) to Settings → Calls.
The IncomingCall audio element reads the setting and applies it as
audioElement.volume before playing, replacing the implicit browser
default of 1.0.

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

src/app/state/settings.ts

@@ -148,6 +148,7 @@ export interface Settings {
   afkTimeoutMinutes: number;
 
   callJoinLeaveSound: 'off' | 'chime' | 'soft' | 'retro';
+  ringtoneVolume: number; // 0–100
 
   seasonalThemeOverride:
     | 'auto'
@@ -242,6 +243,7 @@ const defaultSettings: Settings = {
   afkTimeoutMinutes: 10,
 
   callJoinLeaveSound: 'chime',
+  ringtoneVolume: 70,
 
   seasonalThemeOverride: 'auto',
 };