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/mcp_server/README.md

@@ -0,0 +1,103 @@
+# Voicebox MCP server
+
+Local **Model Context Protocol** server — lets any MCP-aware agent
+(Claude Code, Cursor, Windsurf, VS Code MCP extensions, etc.) speak text
+in your cloned voices, transcribe audio, and browse captures.
+
+The server runs inside the same `uvicorn` process as the rest of Voicebox
+and is mounted at `/mcp` (Streamable HTTP transport).
+
+## Install into your agent
+
+Preferred — direct HTTP:
+
+```json
+{
+  "mcpServers": {
+    "voicebox": {
+      "url": "http://127.0.0.1:17493/mcp",
+      "headers": { "X-Voicebox-Client-Id": "claude-code" }
+    }
+  }
+}
+```
+
+Fallback — stdio shim (when the client doesn't speak HTTP MCP). The
+`voicebox-mcp` binary ships inside the Voicebox.app bundle:
+
+```json
+{
+  "mcpServers": {
+    "voicebox": {
+      "command": "/Applications/Voicebox.app/Contents/MacOS/voicebox-mcp",
+      "env": { "VOICEBOX_CLIENT_ID": "claude-code" }
+    }
+  }
+}
+```
+
+Claude Code one-liner:
+
+```
+claude mcp add voicebox \
+  --transport http \
+  --url http://127.0.0.1:17493/mcp \
+  --header "X-Voicebox-Client-Id: claude-code"
+```
+
+## Tools
+
+| Name | Purpose |
+|---|---|
+| `voicebox.speak`          | Speak text in a voice profile. Returns a generation id you can poll. |
+| `voicebox.transcribe`     | Whisper transcription of a base64 blob or an absolute local path. |
+| `voicebox.list_captures`  | Recent captures (dictation / recording / file) with transcripts. |
+| `voicebox.list_profiles`  | Available voice profiles (cloned + preset). |
+
+All tools resolve voice profiles in this precedence:
+
+1. Explicit `profile` arg (name or id — case-insensitive)
+2. Per-client binding keyed by `X-Voicebox-Client-Id`
+3. `capture_settings.default_playback_voice_id` (global default)
+
+Bindings are managed via `GET|PUT /mcp/bindings` or in the app under
+Settings → MCP.
+
+## Debug with MCP Inspector
+
+```
+npx @modelcontextprotocol/inspector http://127.0.0.1:17493/mcp
+```
+
+Point it at the URL, hit "List tools," call `voicebox.list_profiles`
+first to confirm wiring, then `voicebox.speak` for end-to-end.
+
+## Non-MCP REST surface
+
+`POST /speak` is a thin wrapper on the same code path for callers that
+don't speak MCP (shell scripts, ACP, A2A):
+
+```
+curl -X POST http://127.0.0.1:17493/speak \
+  -H 'Content-Type: application/json' \
+  -H 'X-Voicebox-Client-Id: claude-code' \
+  -d '{"text":"Build complete.","profile":"Morgan"}'
+```
+
+## Code layout
+
+```
+backend/mcp_server/
+├── __init__.py      # re-export mount_into
+├── server.py        # build_mcp_server() + mount_into(app)
+├── tools.py         # @mcp.tool() implementations
+├── context.py       # ClientIdMiddleware + current_client_id ContextVar
+├── resolve.py       # profile resolution precedence
+├── events.py        # pub/sub queue for /events/speak pill SSE
+└── README.md        # you are here
+
+backend/mcp_shim/    # stdio ↔ Streamable-HTTP proxy (see its README)
+```
+
+The package is **`mcp_server`**, not `mcp`, to avoid shadowing the
+installed `mcp` PyPI package that FastMCP imports internally.