feat: add cmd+z undo support for deleting notes

[devin-ai-integration]

Summary

Adds undo functionality for note deletion. When a user deletes a note, they can press Cmd+Z (Mac) / Ctrl+Z (Windows/Linux) or click the "Undo" button in a toast notification to restore it within 5 seconds.

How it works:

• Before deletion, a TinyBase checkpoint captures the current state
• Session data is deleted immediately from TinyBase (for responsive UI)
• Audio file deletion is delayed by 5 seconds to allow undo
• On undo, the checkpoint is restored and the audio deletion is cancelled
• Session persister cleanup is deferred for pending deletes to prevent race conditions

Files changed:

• store/zustand/undo-delete.ts - New Zustand store tracking pending deletions + getPendingDeleteSessionIds() helper
• hooks/useUndoDelete.ts - Hook that registers the mod+z hotkey
• components/undo-delete-toast/index.tsx - Toast UI with undo button
• store/tinybase/store/deleteSession.ts - Extracted deleteSessionData helper
• store/tinybase/persister/session/persister.ts - Modified cleanup to respect pending deletes
• delete.tsx - Updated to use checkpoint-based deletion

Updates since last revision

• Fixed race condition between undo and persister cleanup: sessions awaiting undo are added to keepIds to prevent premature directory deletion
• Fixed toast not appearing: added key prop to AnimatePresence child (required for motion/react to properly track mount/unmount lifecycle)
• Added checkpoints.clear() after undo to clean up checkpoint history

Review & Testing Checklist for Human

• Verify toast appears: Delete a note → confirm toast notification shows at bottom of screen with note title
• Test full undo flow: Delete a note → click Undo or press Cmd+Z → verify note is restored with all data (transcripts, participants, tags)
• Test keyboard shortcut isolation: While editing note content, press Cmd+Z → verify it undoes text changes, not the last deletion
• Test audio preservation: Delete a note with a recording → undo within 5 seconds → verify audio file is still playable
• Test timeout behavior: Delete a note → wait >5 seconds → verify undo is no longer available and audio is deleted

Recommended test plan:

1. Create a note with content and a recording
2. Delete it via the overflow menu
3. Verify toast shows with note title
4. Press Cmd+Z to undo
5. Verify note reappears with content and audio intact
6. Delete again and wait 6 seconds
7. Verify toast disappears and undo no longer works

Notes

Known limitation: If undo is triggered after 5 seconds, the session data cannot be restored (toast is already gone). The audio file is permanently deleted at the 5-second mark.

Not tested locally: Due to Rust toolchain environment issues, local app testing was not possible. TypeScript typecheck passes and CI should verify the build.

Requested by @ComputelessComputer

Link to Devin run: https://app.devin.ai/sessions/364a0344939c4210be93395293bb208e

[devin-ai-integration]

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