Remove MCP tab and per-project MCP UI from frontend

Claude Code manages MCP natively now (`claude mcp add/list/remove`,
`.mcp.json`, `/mcp`), so Triple-C's own MCP server library is redundant.

Deletes components/mcp/, hooks/useMcpServers.ts, the MCP sidebar tab and
rail icon, the per-project enable checkboxes on ProjectCard, the
mcpServers slice of the Zustand store, the four IPC wrappers, and the
McpServer/McpTransportType types.

Rust backend is untouched in this commit; the commands simply become
unreachable. Backend removal and the legacy container/network cleanup
follow separately.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -22,9 +22,6 @@ 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(() => {