feat: batch session archive and delete with multi-select UI

Add multi-select mode to the sidebar session list, enabling users to
archive, restore, or delete multiple sessions at once.

UX:
- Ctrl+Click (Cmd+Click on Mac) any session to enter multi-select mode
- Animated checkboxes stagger-reveal across all visible sessions
- Click to toggle, Shift+Click for range selection
- Running sessions are excluded from selection (safety guard)
- Floating action bar slides up with count pill, Archive, Delete, Cancel
- Batch delete shows a confirmation dialog with hold-to-confirm for 3+
  sessions and a scrollable list of session titles
- Batch archive shows an undo toast with 5-second auto-dismiss
- Escape key or Cancel button exits multi-select mode

Backend:
- batchSetSessionsArchived IPC: single load/mutate/persist cycle for N
  sessions instead of N sequential calls
- batchDeleteSessions IPC: parallel cleanup of scratchpad dirs and SDK
  sessions, then single workspace persist

Accessibility:
- Action bar: role=toolbar, aria-label
- Checkboxes: role=checkbox, aria-checked, keyboard-activatable
- Selection count: aria-live=polite
- Delete dialog: role=alertdialog, aria-modal, focus trap

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

src/main/AryxAppService.ts

@@ -1149,6 +1149,50 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
     return this.persistAndBroadcast(workspace);
   }
 
+  async batchSetSessionsArchived(sessionIds: string[], isArchived: boolean): Promise<WorkspaceState> {
+    const workspace = await this.loadWorkspace();
+    const now = nowIso();
+    for (const sessionId of sessionIds) {
+      const session = workspace.sessions.find((s) => s.id === sessionId);
+      if (session) {
+        session.isArchived = isArchived;
+        session.updatedAt = now;
+      }
+    }
+    return this.persistAndBroadcast(workspace);
+  }
+
+  async batchDeleteSessions(sessionIds: string[]): Promise<WorkspaceState> {
+    const workspace = await this.loadWorkspace();
+    const idsToDelete = new Set(sessionIds);
+
+    // Collect cleanup work before mutating the array
+    const cleanupTasks: Promise<void>[] = [];
+    for (const session of workspace.sessions) {
+      if (!idsToDelete.has(session.id)) continue;
+
+      const scratchpadDirectory = this.resolveScratchpadSessionDirectory(session);
+      if (scratchpadDirectory) {
+        cleanupTasks.push(rm(scratchpadDirectory, { recursive: true, force: true }));
+      }
+      cleanupTasks.push(
+        this.sidecar.deleteSession(session.id).then(() => undefined).catch(() => {
+          // Best-effort — don't fail the deletion if SDK cleanup fails
+        }),
+      );
+    }
+
+    await Promise.allSettled(cleanupTasks);
+
+    workspace.sessions = workspace.sessions.filter((s) => !idsToDelete.has(s.id));
+
+    if (workspace.selectedSessionId && idsToDelete.has(workspace.selectedSessionId)) {
+      workspace.selectedSessionId = workspace.sessions[0]?.id;
+    }
+
+    return this.persistAndBroadcast(workspace);
+  }
+
   async regenerateSessionMessage(sessionId: string, messageId: string): Promise<void> {
     const workspace = await this.loadWorkspace();
     const session = this.requireSession(workspace, sessionId);

src/main/ipc/registerIpcHandlers.ts

@@ -4,6 +4,8 @@ import type { BrowserWindow } from 'electron';
 import { ipcChannels } from '@shared/contracts/channels';
 import type {
   BranchSessionInput,
+  BatchDeleteSessionsInput,
+  BatchSetSessionsArchivedInput,
   CancelSessionTurnInput,
   CommitProjectGitChangesInput,
   CreateSessionInput,
@@ -224,6 +226,12 @@ export function registerIpcHandlers(
   ipcMain.handle(ipcChannels.deleteSession, (_event, input: DeleteSessionInput) =>
     service.deleteSession(input.sessionId),
   );
+  ipcMain.handle(ipcChannels.batchSetSessionsArchived, (_event, input: BatchSetSessionsArchivedInput) =>
+    service.batchSetSessionsArchived(input.sessionIds, input.isArchived),
+  );
+  ipcMain.handle(ipcChannels.batchDeleteSessions, (_event, input: BatchDeleteSessionsInput) =>
+    service.batchDeleteSessions(input.sessionIds),
+  );
   ipcMain.handle(ipcChannels.regenerateSessionMessage, (_event, input: RegenerateSessionMessageInput) =>
     service.regenerateSessionMessage(input.sessionId, input.messageId),
   );