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-tauri/Cargo.toml

@@ -78,3 +78,8 @@ gdk = "0.18"
 [target.'cfg(target_os = "macos")'.dependencies]
 objc2 = "0.6.4"
 objc2-foundation = { version = "0.3.2", default-features = false, features = ["std", "NSString", "NSProcessInfo"] }
+
+[target.'cfg(target_os = "windows")'.dependencies]
+# Phase 4 TTS: PowerShell -EncodedCommand expects UTF-16-LE base64.
+# Windows-only because the other platforms' TTS paths pass text via argv.
+base64 = "0.22"