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/pages/DictationPage.svelte

@@ -17,6 +17,7 @@
   import { FEEDBACK_TIMEOUT_MS } from "$lib/utils/constants.js";
   import { Mic, Loader2, SquareCheck, AlertTriangle } from 'lucide-svelte';
   import EmptyState from '$lib/components/EmptyState.svelte';
+  import SpeakerButton from '$lib/components/SpeakerButton.svelte';
   import { getPreferences } from '$lib/stores/preferences.svelte.js';
   import { bionicReading } from '$lib/actions/bionicReading.js';
   import { measurePreWrap } from '$lib/utils/textMeasure.js';
@@ -1069,6 +1070,9 @@
             <span class="text-[11px] text-text-tertiary">
               {settings.formatMode} · {page.activeProfile === "None" ? "No profile" : page.activeProfile}
             </span>
+            {#if transcript.trim()}
+              <SpeakerButton text={transcript} label="Read transcript aloud" size={12} />
+            {/if}
           </div>
         </div>
       </Card>