feat: add MCP server support with global library and per-project toggles

Add Model Context Protocol (MCP) server configuration support. Users can
define MCP servers globally (new sidebar tab) and enable them per-project.
Enabled servers are injected into containers as MCP_SERVERS_JSON env var
and merged into ~/.claude.json by the entrypoint.

Backend: McpServer model, McpStore (JSON + atomic writes), 4 CRUD commands,
container injection with fingerprint-based recreation detection.
Frontend: MCP sidebar tab, McpPanel/McpServerCard components, useMcpServers
hook, per-project MCP checkboxes in ProjectCard config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/components/layout/Sidebar.test.tsx

@@ -19,6 +19,9 @@ vi.mock("../projects/ProjectList", () => ({
 vi.mock("../settings/SettingsPanel", () => ({
   default: () => <div data-testid="settings-panel">SettingsPanel</div>,
 }));
+vi.mock("../mcp/McpPanel", () => ({
+  default: () => <div data-testid="mcp-panel">McpPanel</div>,
+}));
 
 describe("Sidebar", () => {
   beforeEach(() => {