feat(mcp): local MCP server exposes voicebox.* tools to AI agents

Mounts FastMCP at /mcp (Streamable HTTP) so Claude Code, Cursor,
Windsurf, and the VS Code MCP extensions can call voicebox.speak,
voicebox.transcribe, voicebox.list_captures, and voicebox.list_profiles
against the running Voicebox server.

Backend
- new backend/mcp_server package (tools, middleware, profile resolve,
  pub/sub events); named mcp_server to avoid shadowing the installed mcp
  PyPI package FastMCP imports internally
- app.py migrated from @app.on_event to lifespan= so FastMCP's session
  manager cohabits with Voicebox's startup/shutdown
- new MCPClientBinding table + /mcp/bindings CRUD; ClientIdMiddleware
  reads X-Voicebox-Client-Id into a ContextVar and stamps last_seen_at
- profile resolution precedence: explicit -> per-client binding ->
  capture_settings.default_playback_voice_id
- POST /speak REST wrapper for non-MCP callers (shell, ACP, A2A)
- GET /events/speak SSE broadcasts speak-start / speak-end so the pill
  surfaces agent-initiated speech
- backend/mcp_shim proxy (plain httpx) for stdio-only MCP clients
- PyInstaller spec updates + new --shim build target (~18 MB)

Frontend
- Settings -> MCP page with HTTP / stdio / claude-mcp-add copy snippets,
  default voice picker, per-client bindings table, connection status
- useMCPBindings, useSpeakEvents hooks
- CapturePill gains 'speaking' state; DictateWindow subscribes to SSE
  and emits dictate:show so the Rust side surfaces the pill window

Native
- tauri.conf.json externalBin now includes voicebox-mcp
- show_dictate_window helper + dictate:show listener in main.rs
- (also in this commit: InputMonitoringGate UX, hotkey_monitor tweaks,
  landing footer/navbar updates, new overview docs for captures /
  dictation / mcp-server / voice-personalities)

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/build_binary.py

@@ -295,6 +295,28 @@ def build_server(cuda=False):
             "unidic_lite",
             "--hidden-import",
             "loguru",
+            # MCP server — Streamable-HTTP endpoint and the 4 voicebox.* tools.
+            # FastMCP pulls in a chain of deps (mcp, cyclopts, openapi-pydantic,
+            # etc.) that don't auto-discover cleanly under PyInstaller, so we
+            # collect them whole. Small compared to torch.
+            "--hidden-import",
+            "backend.mcp_server",
+            "--hidden-import",
+            "backend.mcp_server.server",
+            "--hidden-import",
+            "backend.mcp_server.tools",
+            "--hidden-import",
+            "backend.mcp_server.context",
+            "--hidden-import",
+            "backend.mcp_server.resolve",
+            "--hidden-import",
+            "backend.mcp_server.events",
+            "--collect-all",
+            "fastmcp",
+            "--collect-all",
+            "mcp",
+            "--hidden-import",
+            "sse_starlette",
         ]
     )
 
@@ -447,12 +469,108 @@ def build_server(cuda=False):
     logger.info("Binary built in %s", backend_dir / "dist" / binary_name)
 
 
+def build_shim():
+    """Build the voicebox-mcp stdio shim as a tiny standalone binary.
+
+    This is the bridge for MCP clients that only speak stdio — it proxies
+    JSON-RPC to the main voicebox-server's /mcp endpoint. Keep it small: no
+    torch, no ML deps, just httpx + asyncio.
+    """
+    backend_dir = Path(__file__).parent
+
+    args = [
+        "mcp_shim/__main__.py",
+        "--onefile",
+        "--name",
+        "voicebox-mcp",
+        # Stdio-only — no console hiding needed on Windows since the parent
+        # MCP client is spawning this as a child process and wants stdio.
+        "--hidden-import",
+        "backend.mcp_shim",
+        "--hidden-import",
+        "backend.mcp_shim.__main__",
+        "--hidden-import",
+        "httpx",
+        "--hidden-import",
+        "httpx._transports.default",
+        "--hidden-import",
+        "anyio",
+        # Exclude everything heavy that httpx/asyncio don't actually need so
+        # the binary stays tiny (~15 MB instead of ~400 MB).
+        "--exclude-module",
+        "torch",
+        "--exclude-module",
+        "transformers",
+        "--exclude-module",
+        "mlx",
+        "--exclude-module",
+        "mlx_audio",
+        "--exclude-module",
+        "qwen_tts",
+        "--exclude-module",
+        "chatterbox",
+        "--exclude-module",
+        "zipvoice",
+        "--exclude-module",
+        "tada",
+        "--exclude-module",
+        "kokoro",
+        "--exclude-module",
+        "misaki",
+        "--exclude-module",
+        "spacy",
+        "--exclude-module",
+        "librosa",
+        "--exclude-module",
+        "numba",
+        "--exclude-module",
+        "numpy",
+        "--exclude-module",
+        "pedalboard",
+        "--exclude-module",
+        "fastapi",
+        "--exclude-module",
+        "uvicorn",
+        "--exclude-module",
+        "sqlalchemy",
+        "--exclude-module",
+        "fastmcp",
+        "--exclude-module",
+        "mcp",
+    ]
+
+    dist_dir = str(backend_dir / "dist")
+    build_dir = str(backend_dir / "build")
+    args.extend(
+        [
+            "--distpath",
+            dist_dir,
+            "--workpath",
+            build_dir,
+            "--noconfirm",
+            "--clean",
+        ]
+    )
+
+    os.chdir(backend_dir)
+    PyInstaller.__main__.run(args)
+    logger.info("Shim built: %s", backend_dir / "dist" / "voicebox-mcp")
+
+
 if __name__ == "__main__":
-    parser = argparse.ArgumentParser(description="Build voicebox-server binary")
+    parser = argparse.ArgumentParser(description="Build voicebox binaries")
     parser.add_argument(
         "--cuda",
         action="store_true",
         help="Build CUDA-enabled binary (voicebox-server-cuda)",
     )
+    parser.add_argument(
+        "--shim",
+        action="store_true",
+        help="Build the voicebox-mcp stdio shim binary instead of the server",
+    )
     cli_args = parser.parse_args()
-    build_server(cuda=cli_args.cuda)
+    if cli_args.shim:
+        build_shim()
+    else:
+        build_server(cuda=cli_args.cuda)