Restructure into asr/tts packages and add TTS tab skeleton

Split the single ASR gateway into app/main.py (entrypoint) + app/core
(shared env helpers) + app/asr (all existing ASR logic, unchanged
behavior) + app/tts (placeholder router/config, no engine yet), so a
real TTS backend can be dropped in later without reshuffling ASR code.
API moved under /asr/* and /tts/* prefixes; UI gained a top-level
ASR/TTS tab switcher built on a reusable nested .tab-group mechanism,
with JS split into common.js/asr.js/tts.js.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/ui/assets/js/tts.js

@@ -0,0 +1,15 @@
+// TTS tab is a structural placeholder until a synthesis backend is wired in.
+// This just confirms the /tts router is reachable; the UI itself is a static
+// "coming soon" card (see index.html).
+
+async function loadTtsConfig() {
+  try {
+    const r = await fetch('/tts/config');
+    const cfg = await r.json();
+    console.log('[tts] config', cfg);
+  } catch (e) {
+    console.warn('[tts] config fetch failed', e);
+  }
+}
+
+loadTtsConfig();