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/shared/contracts/channels.ts

@@ -49,6 +49,8 @@ export const ipcChannels = {
   setSessionPinned: 'sessions:set-pinned',
   setSessionArchived: 'sessions:set-archived',
   deleteSession: 'sessions:delete',
+  batchSetSessionsArchived: 'sessions:batch-set-archived',
+  batchDeleteSessions: 'sessions:batch-delete',
   regenerateSessionMessage: 'sessions:regenerate-message',
   editAndResendSessionMessage: 'sessions:edit-and-resend-message',
   sendSessionMessage: 'sessions:send-message',