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/preload/index.ts

@@ -67,6 +67,8 @@ const api: ElectronApi = {
   setSessionPinned: (input) => ipcRenderer.invoke(ipcChannels.setSessionPinned, input),
   setSessionArchived: (input) => ipcRenderer.invoke(ipcChannels.setSessionArchived, input),
   deleteSession: (input) => ipcRenderer.invoke(ipcChannels.deleteSession, input),
+  batchSetSessionsArchived: (input) => ipcRenderer.invoke(ipcChannels.batchSetSessionsArchived, input),
+  batchDeleteSessions: (input) => ipcRenderer.invoke(ipcChannels.batchDeleteSessions, input),
   regenerateSessionMessage: (input) => ipcRenderer.invoke(ipcChannels.regenerateSessionMessage, input),
   editAndResendSessionMessage: (input) => ipcRenderer.invoke(ipcChannels.editAndResendSessionMessage, input),
   sendSessionMessage: (input) => ipcRenderer.invoke(ipcChannels.sendSessionMessage, input),