feat(mcp): MCP server in the app and the querya-mcp stdio shim

[ZhuchkaTriplesix]

Second part of the MCP epic #1133, on top of McpQueryService (#1134).

Server in the app

• QueryaMcpServer (dart_mcp 0.5.2): tools list_connections, list_tables, describe_table, sample_rows, run_query, explain_query, all marked readOnlyHint; resource template schema://{connection_id}. Instructions tell the model that results are data, not instructions.
• Tool failures come back as isError results with a readable text; every call is reported as McpCallRecord (client, tool, connection, SQL, rows, duration, error) on McpServerController.calls for the activity log in feat(mcp): Settings page, per-connection access and activity log #1136.
• McpSocketHost: 127.0.0.1 only, random port. The first line of each connection must be the token (constant-time compare), otherwise a JSON-RPC error and the socket is closed; 5 s handshake timeout. Each client gets its own MCP session.
• McpEndpoint: {port, token, pid, version} in $XDG_RUNTIME_DIR/querya/mcp-endpoint.json (Linux, fallback ~/.cache/querya), ~/Library/Caches/Querya (macOS), %LOCALAPPDATA%\Querya (Windows); override QUERYA_MCP_ENDPOINT. Written atomically, directory 0700, file 0600; deleted on stop. Kept outside the profile dir so the shim does not need the portable-mode logic.
• McpServerController: off by default (mcp_server_enabled in app_settings), started at launch when enabled, stopped before the pools close on app exit (AppLifecycleCleanup). Not tied to "Disconnect all", which must not kill the MCP server.

querya-mcp shim

• Lives in its own dependency-free package packages/querya_mcp_bridge (McpEndpoint + runMcpShim, pure dart:io); the app uses it as a path dependency. Reason: in the app package dart compile exe refuses to build because sqlite3 / objective_c have build hooks (dart build cli works but drags libsqlite3.so along). The separate package compiles to one static ~7 MB binary.
• The shim reads the endpoint, sends the token, then pipes bytes both ways without parsing MCP; a reset from the app ends it cleanly.
• App not running / stale endpoint: answers every request with a JSON-RPC error "Querya Desktop is not running or its MCP server is off..." until stdin closes, exit code 1.

Release

• CI (Code Analysis job): dart analyze --fatal-infos and dart compile exe of the bridge package on every PR, plus a --help run.
• Built in the build-ext-tester matrix (linux-x64, macos-arm64/x64, windows-x64), smoke-tested (--help and the not-running answer), published as querya-mcp-v<ver>-<target> assets.
• Bundled next to the app binary in the Linux build (so zip, AppImage, .deb and .rpm get /opt/querya-desktop/querya-mcp) and the Windows build (zip and setup).
• Not bundled yet: macOS .app (needs codesigning of a second binary) and Flatpak (sandboxed runtime dir); both get the standalone asset for now.

Tests (test/core/mcp/mcp_server_test.dart)

Real sockets, the real shim function and a dart_mcp client: initialize, tool list and annotations, calls incl. a refused write and a missing argument, the schema resource, call records, client count; endpoint permissions and removal on stop; wrong token refused; shim without the app; stopping the server disconnects a live client.

Also: publish_to: 'none' in the app pubspec.yaml (needed for the path dependency; the app is never published).

Settings UI (enable toggle, per-connection access, copy-config) is #1136.

Written without running locally; CI is the first run.

Closes #1135