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/core/config.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import os
+
+
+def env_str(name: str, default: str = "") -> str:
+    return os.getenv(name, default)
+
+
+def env_int(name: str, default: int) -> int:
+    raw = os.getenv(name)
+    if raw is None or raw == "":
+        return default
+    return int(raw)
+
+
+def env_float(name: str, default: float) -> float:
+    raw = os.getenv(name)
+    if raw is None or raw == "":
+        return default
+    return float(raw)
+
+
+def env_bool(name: str, default: bool = False) -> bool:
+    raw = os.getenv(name)
+    if raw is None:
+        return default
+    return raw.strip().lower() in {"1", "true", "yes", "on"}