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/tts/router.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from typing import Any, Dict, List
+
+from fastapi import APIRouter, HTTPException
+
+from tts.config import DEFAULT_TTS_BACKEND
+
+router = APIRouter()
+
+
+@router.get("/config")
+def config() -> Dict[str, Any]:
+    return {
+        "status": "not_implemented",
+        "default_backend": DEFAULT_TTS_BACKEND,
+        "backends": {},
+        "message": "TTS 백엔드가 아직 연동되지 않았습니다.",
+    }
+
+
+@router.get("/history")
+def list_history() -> List[Dict[str, Any]]:
+    return []
+
+
+@router.post("/synthesize")
+def synthesize() -> None:
+    raise HTTPException(status_code=501, detail="TTS 합성 기능은 아직 구현되지 않았습니다.")