feat: add MCP OAuth authentication frontend wiring and banner

- Add McpOauthRequiredEvent and McpOauthStaticClientConfigEvent to sidecar contracts
- Add PendingMcpAuthRecord domain type with status tracking (pending/authenticating/failed/done)
- Add pendingMcpAuth field to SessionRecord for session-level auth state
- Wire onMcpOAuthRequired callback through sidecarProcess, runTurnPending, and AryxAppService
- Handle mcp-oauth-required events: set session pendingMcpAuth, clear on turn finalize/cancel
- Add dismissSessionMcpAuth IPC channel with preload binding and handler registration
- Create McpAuthBanner component (amber-themed, shows server name/URL, dismiss button)
- Integrate McpAuthBanner into ChatPane with placeholder text and App.tsx callback
- Update test fixtures for new RunTurnPendingCommand.onMcpOAuthRequired field

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

src/shared/contracts/sidecar.ts

@@ -244,6 +244,23 @@ export interface UserInputRequestedEvent {
   allowFreeform?: boolean;
 }
 
+export interface McpOauthStaticClientConfigEvent {
+  clientId: string;
+  publicClient?: boolean;
+}
+
+export interface McpOauthRequiredEvent {
+  type: 'mcp-oauth-required';
+  requestId: string;
+  sessionId: string;
+  oauthRequestId: string;
+  agentId?: string;
+  agentName?: string;
+  serverName: string;
+  serverUrl: string;
+  staticClientConfig?: McpOauthStaticClientConfigEvent;
+}
+
 export interface ExitPlanModeRequestedEvent {
   type: 'exit-plan-mode-requested';
   requestId: string;
@@ -276,6 +293,7 @@ export type SidecarEvent =
   | AgentActivityEvent
   | ApprovalRequestedEvent
   | UserInputRequestedEvent
+  | McpOauthRequiredEvent
   | ExitPlanModeRequestedEvent
   | CommandErrorEvent
   | CommandCompleteEvent;