feat(ui B.1 #20): sound cues on start / stop / complete via Web Audio

Hands-off feedback for recording lifecycle: a short C5 pitch at
record start, a falling G4 at record stop, and a staggered C5–E5–G5
major third when the finalise flow completes. Synthesised at runtime
via the Web Audio API (OscillatorNode + linear-ramped GainNode
envelope) rather than shipping WAV assets — keeps the binary size
flat and lets us tweak timbre without touching bundled files.

Off by default. Settings → Output exposes the toggle with a volume
slider (0–100%, default 15%) and a "Test" button that plays the
completion cue so the user can confirm loudness without recording.

Hooked at three call sites in DictationPage:
- playStartCue after page.recording = true in startRecording
- playStopCue at the top of stopRecording
- playCompleteCue just before `saved = true` at the end of
  finaliseTranscription's transcript-present branch

All three no-op when settings.soundCues is false. The Web Audio
context is lazily constructed on first cue (most browsers suspend
it until a user gesture — Tauri's webview inherits that). If the
AudioContext can't be built we silently drop the cue rather than
throwing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/pages/DictationPage.svelte

@@ -6,6 +6,7 @@
   import { getCurrentWindow } from "@tauri-apps/api/window";
   import { page, settings, templates, profiles, addToHistory, addTask, tasks } from "$lib/stores/page.svelte.js";
   import { markGenerating, markGenerationDone } from "$lib/stores/llmStatus.svelte.js";
+  import { playStartCue, playStopCue, playCompleteCue } from "$lib/utils/sounds.js";
   import { profilesStore } from "$lib/stores/profiles.svelte.ts";
   import { toasts } from "$lib/stores/toasts.svelte.js";
   import Card from "$lib/components/Card.svelte";
@@ -358,6 +359,9 @@
       page.statusColor = "#e87171";
       timerInterval = setInterval(updateTimer, 1000);
 
+      // Start cue (brief item #20). Off by default; volume user-set.
+      playStartCue(settings.soundCueVolume, settings.soundCues);
+
       // Preview overlay: if the user opted in, reset any leftover state
       // from a prior run and open the window when the main window is not
       // focused (i.e. the user is dictating into some other app).
@@ -389,6 +393,9 @@
     page.status = "Finalising...";
     page.statusColor = "#e8c86e";
 
+    // Stop cue (brief item #20). "Recording over, finalising now."
+    playStopCue(settings.soundCueVolume, settings.soundCues);
+
     try {
       const activityBeforeStop = lastLiveActivityAt;
       drainingSessionId = sessionId;
@@ -570,6 +577,11 @@
         page.taskSidebarOpen = true;
       }
 
+      // Complete cue (brief item #20) — major-third arpeggio. Plays
+      // after the full finalise flow so the user hears "done"
+      // aligned with the history entry + paste landing.
+      playCompleteCue(settings.soundCueVolume, settings.soundCues);
+
       saved = true;
       setTimeout(() => { saved = false; extractedCount = 0; }, 4000);
     } else if (settings.transcriptionPreview) {