feat: implement MCP OAuth 2.1 flow, token storage, and injection

- Create mcpTokenStore: in-memory token storage keyed by normalized server URL
  with automatic expiry checking
- Create mcpOAuthService: full OAuth 2.1 + PKCE flow implementation
  - Protected Resource Metadata discovery (RFC 9728)
  - Authorization Server Metadata fetch (RFC 8414)
  - Dynamic Client Registration (RFC 7591) when no static client ID
  - PKCE S256 code challenge generation
  - Local HTTP callback server for auth code receipt
  - Browser-based consent via Electron shell.openExternal
  - Authorization code to token exchange
- Add startSessionMcpAuth IPC channel and handler to trigger OAuth flow
- Inject stored OAuth tokens as Authorization headers in buildRunTurnToolingConfig
- Update McpAuthBanner with 'Authenticate in browser' button and loading state
- Add tests for token store (7 tests) and token injection (3 tests)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/channels.ts

@@ -30,6 +30,7 @@ export const ipcChannels = {
   setSessionInteractionMode: 'sessions:set-interaction-mode',
   dismissSessionPlanReview: 'sessions:dismiss-plan-review',
   dismissSessionMcpAuth: 'sessions:dismiss-mcp-auth',
+  startSessionMcpAuth: 'sessions:start-mcp-auth',
   querySessions: 'sessions:query',
   updateSessionModelConfig: 'sessions:update-model-config',
   selectProject: 'selection:project',