fitness: TTS volume control, audio ducking, and workout start/finish announcements

Add TTS volume slider (default 80%) and audio duck toggle to voice
guidance settings. Announce "Workout started" when TTS initializes and
speak a full workout summary (time, distance, avg pace) on finish.
The finish summary reuses the existing TTS instance via handoff so it
plays fully without blocking the completion screen.

src/lib/js/gps.svelte.ts

@@ -26,6 +26,8 @@ export interface VoiceGuidanceConfig {
 	metrics: string[];
 	language: string;
 	voiceId?: string;
+	ttsVolume?: number; // 0.0–1.0, relative TTS volume
+	audioDuck?: boolean; // duck other audio during TTS
 	intervals?: IntervalStep[];
 }