feat: add turn cancellation UI and main process wiring

Adds cancel-turn IPC channel, SidecarClient.cancelTurn(), TurnCancelledError,
EryxAppService.cancelSessionTurn(), finalizeCancelledTurn(), stop button in
ChatPane, and cancelled run status throughout the run timeline.

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

src/shared/contracts/channels.ts

@@ -24,6 +24,7 @@ export const ipcChannels = {
   setSessionPinned: 'sessions:set-pinned',
   setSessionArchived: 'sessions:set-archived',
   sendSessionMessage: 'sessions:send-message',
+  cancelSessionTurn: 'sessions:cancel-turn',
   resolveSessionApproval: 'sessions:resolve-approval',
   querySessions: 'sessions:query',
   updateScratchpadSessionConfig: 'sessions:update-scratchpad-config',

src/shared/contracts/ipc.ts

@@ -26,6 +26,10 @@ export interface SendSessionMessageInput {
   content: string;
 }
 
+export interface CancelSessionTurnInput {
+  sessionId: string;
+}
+
 export interface ResolveSessionApprovalInput {
   sessionId: string;
   approvalId: string;
@@ -120,6 +124,7 @@ export interface ElectronApi {
   setSessionPinned(input: SetSessionPinnedInput): Promise<WorkspaceState>;
   setSessionArchived(input: SetSessionArchivedInput): Promise<WorkspaceState>;
   sendSessionMessage(input: SendSessionMessageInput): Promise<void>;
+  cancelSessionTurn(input: CancelSessionTurnInput): Promise<void>;
   resolveSessionApproval(input: ResolveSessionApprovalInput): Promise<WorkspaceState>;
   updateScratchpadSessionConfig(input: UpdateScratchpadSessionConfigInput): Promise<WorkspaceState>;
   querySessions(input: QuerySessionsInput): Promise<SessionQueryResult[]>;

src/shared/contracts/sidecar.ts

@@ -79,6 +79,12 @@ export interface RunTurnCommand {
   tooling?: RunTurnToolingConfig;
 }
 
+export interface CancelTurnCommand {
+  type: 'cancel-turn';
+  requestId: string;
+  targetRequestId: string;
+}
+
 export interface ResolveApprovalCommand {
   type: 'resolve-approval';
   requestId: string;
@@ -90,6 +96,7 @@ export type SidecarCommand =
   | DescribeCapabilitiesCommand
   | ValidatePatternCommand
   | RunTurnCommand
+  | CancelTurnCommand
   | ResolveApprovalCommand;
 
 export interface RunTurnLocalMcpServerConfig {
@@ -157,6 +164,7 @@ export interface TurnCompleteEvent {
   requestId: string;
   sessionId: string;
   messages: ChatMessageRecord[];
+  cancelled?: boolean;
 }
 
 export type AgentActivityType = 'thinking' | 'tool-calling' | 'handoff' | 'completed';