Add server-side Piper read-aloud with voice picker

Reader read-aloud now streams neural WAV audio from a new POST /api/tts
endpoint that shells out to Piper, instead of the browser's Web Speech
voice. Each paragraph is synthesized on demand with the next one
prefetched during playback, keeping the existing highlight/scroll sync.

Voices are configured under [web.tts] (piper binary + voices_dir + a
labelled voice list) and exposed to the client as window.PETE_TTS; the
reader gets a Voice selector in the Aa menu, persisted per-device. Still
a signed-in-only perk and gated on auth.

internal/web/templates/layout.html

@@ -246,6 +246,10 @@
                   <button type="button" data-paper="sepia">Sepia</button>
                 </div>
               </div>
+              <div class="pete-reader-typerow pete-reader-voicerow" data-reader-voicerow hidden>
+                <span class="pete-reader-typelabel">Voice</span>
+                <select class="pete-reader-voice" data-reader-voice aria-label="Read-aloud voice"></select>
+              </div>
             </div>
           </div>
           <button type="button" data-reader-bookmark class="pete-reader-btn" style="display:none" title="Bookmark (b)" aria-label="Bookmark this story" aria-pressed="false">🔖 Save</button>
@@ -305,6 +309,7 @@
     window.PETE_USER = {{if .User}}{ name: {{.User.Display}}, email: {{.User.Email}} }{{else}}null{{end}};
     window.PETE_PREFS = {{.UserPrefs}};
     window.PETE_PUSH = {{if .PushEnabled}}{ enabled: true, publicKey: {{.PushPublicKey}} }{{else}}null{{end}};
+    window.PETE_TTS = {{.TTS}};
   </script>
   <script src="/static/js/prefs.js" defer></script>
   <script src="/static/js/weather.js" defer></script>