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/database/models.py

@@ -196,6 +196,12 @@ class CaptureSettings(Base):
     preserve_technical = Column(Boolean, nullable=False, default=True)
     allow_auto_paste = Column(Boolean, nullable=False, default=True)
     default_playback_voice_id = Column(String, nullable=True)
+    # Default OFF — opting in is what triggers the macOS Input Monitoring TCC
+    # prompt. We deliberately don't spawn the global keyboard tap until the
+    # user flips this on so a fresh-install user doesn't see a scary
+    # "Voicebox would like to receive keystrokes from any application" dialog
+    # before they've even opened the Captures tab.
+    hotkey_enabled = Column(Boolean, nullable=False, default=False)
     # Lists of rdev::Key variant names (e.g. "MetaRight", "AltGr"). Right-hand
     # modifiers by default so they don't collide with left-hand system
     # shortcuts (Cmd+Opt+I devtools, Cmd+Opt+Esc force-quit).
@@ -221,6 +227,29 @@ class GenerationSettings(Base):
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
 
 
+class MCPClientBinding(Base):
+    """Per-MCP-client settings (voice profile, engine, intent).
+
+    Lets users bind distinct voices to distinct agents — e.g. Claude Code
+    speaks in "Morgan," Cursor in "Scarlett." The MCP client identifies
+    itself via the ``X-Voicebox-Client-Id`` HTTP header; direct-HTTP
+    clients set it in their MCP config's ``headers`` block, the stdio
+    shim forwards it from the ``VOICEBOX_CLIENT_ID`` env var.
+    """
+
+    __tablename__ = "mcp_client_bindings"
+
+    client_id = Column(String, primary_key=True)
+    label = Column(String, nullable=True)  # display name
+    profile_id = Column(String, ForeignKey("profiles.id"), nullable=True)
+    default_engine = Column(String, nullable=True)
+    # "respond" | "rewrite" | "compose" — null means plain TTS (no LLM transform).
+    default_intent = Column(String, nullable=True)
+    last_seen_at = Column(DateTime, nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
 class Capture(Base):
     """A single voice input capture (dictation, recording, or uploaded file).