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/SettingsPage.svelte

@@ -1656,6 +1656,34 @@
                   />
                 </div>
               {/if}
+              <Toggle
+                bind:checked={settings.soundCues}
+                label="Sound cues on start / stop / complete"
+                description="Short synthesised tones for eyes-off feedback. Synthesised locally via Web Audio — no assets bundled."
+              />
+              {#if settings.soundCues}
+                <div class="ml-[50px] mt-2 mb-1 animate-fade-in flex items-center gap-3">
+                  <label for="sound-cue-volume" class="text-[11px] text-text-tertiary uppercase tracking-wider">Volume</label>
+                  <input
+                    id="sound-cue-volume"
+                    type="range"
+                    min="0"
+                    max="1"
+                    step="0.05"
+                    bind:value={settings.soundCueVolume}
+                    class="w-[160px] accent-accent"
+                    data-no-transition
+                  />
+                  <span class="text-[11px] text-text-secondary w-[34px]">{Math.round(settings.soundCueVolume * 100)}%</span>
+                  <button
+                    class="text-[11px] text-accent hover:text-accent-hover"
+                    onclick={async () => {
+                      const mod = await import("$lib/utils/sounds.js");
+                      mod.playCompleteCue(settings.soundCueVolume, true);
+                    }}
+                  >Test</button>
+                </div>
+              {/if}
               <Toggle bind:checked={settings.includeTimestamps} label="Include timestamps in exports" />
               <Toggle
                 bind:checked={settings.saveAudio}