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/renderer/components/ChatPane.tsx

@@ -44,6 +44,7 @@ interface ChatPaneProps {
   onSetInteractionMode?: (mode: InteractionMode) => void;
   onDismissPlanReview?: () => void;
   onDismissMcpAuth?: () => void;
+  onAuthenticateMcp?: () => void;
   onUpdateSessionModelConfig?: (config: {
     model: string;
     reasoningEffort?: ReasoningEffort;
@@ -66,6 +67,7 @@ export function ChatPane({
   onSetInteractionMode,
   onDismissPlanReview,
   onDismissMcpAuth,
+  onAuthenticateMcp,
   onUpdateSessionModelConfig,
   onUpdateSessionTooling,
   onUpdateSessionApprovalSettings,
@@ -145,6 +147,10 @@ export function ChatPane({
     onDismissMcpAuth?.();
   }
 
+  function handleAuthenticateMcp() {
+    onAuthenticateMcp?.();
+  }
+
   async function handleSessionModelConfigChange(config: {
     model: string;
     reasoningEffort?: ReasoningEffort;
@@ -414,6 +420,7 @@ export function ChatPane({
             <div className="mb-3">
               <McpAuthBanner
                 mcpAuth={pendingMcpAuth}
+                onAuthenticate={handleAuthenticateMcp}
                 onDismiss={handleDismissMcpAuth}
               />
             </div>