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/types/app.ts

@@ -70,6 +70,13 @@ export interface SettingsState {
    * the user explicitly opts in.
    */
   matchMyEnergy: boolean;
+  /**
+   * Phase 4 Read Page Aloud: OS-native TTS. `voice` is the platform's
+   * voice id (e.g. macOS `Alex`), or `null` for system default.
+   * `rate` is 0.5..2.0 where 1.0 is the synth's normal speed.
+   */
+  ttsVoice: string | null;
+  ttsRate: number;
 }
 
 export interface Profile {