[FEATURE] Implement undo/redo functionality

[rohitkumarnaidu]

Feature Description

There is no undo/redo functionality. Accidental deletions require manual rebuilding.

Proposed Solution

1. History stack storing snapshots of blocks state array
2. Ctrl+Z (undo) and Ctrl+Shift+Z (redo) keyboard shortcuts
3. Undo/redo buttons in toolbar
4. Limit to last 50 states to prevent memory issues

Technical Approach

Create a useHistory custom hook wrapping useState with past and future stacks.

Why This Matters

• Undo/redo is the Gemini AI Integration Advertised but Not Implemented #1 expected feature in any editor
• Prevents frustration from accidental edits
• Encourages experimentation

[rohitkumarnaidu]

Hey maintainer, I'd love to work on this issue and submit a PR. Could you please assign it to me?

[rohitkumarnaidu]

Hey maintainer, I'd love to work on this issue and submit a PR. Could you please assign it to me?