feat(tts): Phase 4 — Read Page Aloud with OS-native voices

Platform-dispatched TTS (spd-say + espeak-ng fallback on Linux, say on
macOS, PowerShell System.Speech on Windows) with a shared SpeakerButton
component. Tap to speak, tap again to stop; only one button speaks at
a time so two surfaces don't talk over each other. Text always travels
via argv (or a PowerShell here-string delivered through -EncodedCommand
on Windows) so user content never enters a shell string.

Mount points: DictationPage transcript footer, transcript viewer header,
per-step in MicroSteps. Settings gains a "Read aloud" accordion with
voice picker (lazy-loaded from the OS synth), rate slider 0.5-2.0x,
and a British-English test utterance.

Rust tests cover rate mapping, NaN handling, and Windows here-string
terminator safety. No pause/resume, no SSML, no cloud voices — that
stays out of scope per the Layer-1 roadmap.

src/lib/stores/page.svelte.ts

@@ -71,6 +71,8 @@ const defaults: SettingsState = {
   microphoneDevice: "",
   currentEnergy: null,
   matchMyEnergy: false,
+  ttsVoice: null,
+  ttsRate: 1.0,
 };
 
 function canUseStorage(): boolean {

src/lib/stores/speaker.svelte.ts

@@ -0,0 +1,10 @@
+// Phase 4 Read Page Aloud: tracks which SpeakerButton instance is
+// currently driving TTS. Only one speaker is active at a time — when
+// a second button starts speech, the first reverts its icon via the
+// `$derived` subscription in SpeakerButton.
+
+export const activeSpeaker = $state<{ id: string | null }>({ id: null });
+
+export function setActiveSpeaker(id: string | null): void {
+  activeSpeaker.id = id;
+}