feat(audio): add offline local neural voice via Piper (sv + en)

Announcements can use Piper, a free local neural TTS, instead of the browser's system
voice. No account, no paid service, and it keeps working with no internet at the track.

- npm run tts:install (or Settings -> Audio -> Install) downloads the standalone Piper
  release for Linux/Windows/macOS plus Swedish and English voices into piper/ once
- server/tts.js generates each line once, one at a time, and caches it as WAV under
  data/tts/; a new line takes ~0.3 s with the standalone build
- src/piper_speech.js plays it and falls back to the browser voice if Piper is missing,
  errors, or takes over 2.5 s, so no announcement is lost and timing is never held up
- "Pre-generate phrases and driver names" renders every fixed phrase and name in both
  languages before race day

Browser voice stays the default. Generation routes fall under the admin PIN.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/settings.js

@@ -1,5 +1,6 @@
 import { decoderDiagnosticsMarkup, mountDecoderDiagnostics } from "./decoder_diagnostics_view.js";
 import { mountAuthSettings } from "./auth_view.js";
+import { mountTtsSettings } from "./tts_settings_view.js";
 
 let activeSettingsTab = "connection";
 
@@ -233,6 +234,10 @@ export function renderSettings(deps) {
     </div>
 
     <div data-settings-panel="audio">
+    <section class="panel mt-16">
+      <div class="panel-header"><h3>${t("tts.title")}</h3></div>
+      <div class="panel-body" id="ttsSettings"></div>
+    </section>
     <section class="panel mt-16">
       <div class="panel-header"><h3>${t("audio.race_sounds")}</h3></div>
       <div class="panel-body form-grid cols-4 settings-grid settings-grid-toggles">
@@ -641,6 +646,7 @@ Erik Eriksson;654321;4WD;Associated;;</code></pre>
   });
   applySettingsTab(settingsTabButtons.some((button) => button.dataset.settingsTab === activeSettingsTab) ? activeSettingsTab : "connection");
   mountAuthSettings(document.getElementById("authSettings"), { t, escapeHtml });
+  mountTtsSettings(document.getElementById("ttsSettings"), { state, t, escapeHtml, saveState, speakText });
 
   const applyRaceAudioFormValues = (form) => {
     const events = {};