feat(mcp): Settings page, per-connection access and activity log

[ZhuchkaTriplesix]

Third part of the MCP epic #1133: the user-facing side of the server from #1140.

Preferences → MCP Server (new category)

• Enable MCP server switch (off by default) with live status: "Running on 127.0.0.1: · N clients connected" / stopped / start error.
• Shared connections: every PostgreSQL / MySQL / SQLite connection with its own switch, off by default (McpAccessStore). Production connections carry a PROD badge and a warning line. Empty state when there are no SQL connections.
• Connect a client: "Copy … config" for Claude Desktop, Cursor, VS Code and a generic stdio snippet. The command is the bundled querya-mcp next to the app binary; when it is not bundled (macOS, Flatpak, dev builds), the hint says to download it and the snippet has a placeholder. No token or secret in the snippets.
• Regenerate token: restarts the server with a new token; clients reconnect on their next start.
• Recent calls: last 50 calls (time, client, tool, connection, SQL, rows / duration or the error in the destructive color), refreshed after each call; Clear.

Built from UI kit pieces (PreferencesSwitchRow, QueryaActionButton, QueryaBadge, QueryaEmptyState, showAppToast).

Activity log storage

• New mcp_activity table (LocalDb schema v11, created on fresh installs and on upgrade), capped at 200 rows, SQL cut at 2000 chars, connection name stored as a snapshot.
• Only the app's McpServerController.instance persists calls; other instances (tests) do not touch LocalDb.

Deviation from the issue: calls are not written to mutation_audit with a new mcp source. That trail lists executed data / schema changes; MCP is read-only, so every entry would be noise. The activity log is the MCP audit.

Tests

• preferences_mcp_section_test.dart: SQL-only list with shared state and toggling, PROD badge, enabling and status text, regenerate, clipboard content of the copy buttons (path, format, no token), recent calls incl. errors and Clear, empty state.
• mcp_client_config_and_activity_test.dart: snippet formats, bundled shim lookup, mcp_activity round trip, newest-first order, cap, clear.

Written without running locally; CI is the first run.

Closes #1136