feat(design-mode): wire ⌘Z / ⌘⇧Z keyboard shortcuts for Undo/Redo

[Delqhi]

Problem

Issue #52 (Undo/Redo) was closed in #58 with the backend logic done (JSONL stacks, /api/workspace/design-history POST endpoint, pushEntry in design-edit handler). But the keyboard shortcuts (⌘Z / ⌘⇧Z) in the Design Mode preview iframe are not wired.

Goal

When the user is in Design Mode and focused on the preview iframe, pressing:

• ⌘Z (macOS) / Ctrl+Z (Linux/Windows) → triggers POST /api/workspace/design-history { action: 'undo' }
• ⌘⇧Z (macOS) / Ctrl+Shift+Z (Linux/Windows) → triggers POST /api/workspace/design-history { action: 'redo' }

Acceptance Criteria

• Pressing ⌘Z in the preview iframe undoes the last applied change
• Pressing ⌘⇧Z redoes the last undone change
• Shortcuts work on both macOS (⌘) and Linux/Windows (Ctrl)
• Shortcuts are scoped to Design Mode only (not when typing in chat or settings)
• Visual feedback when undo/redo is triggered (toast, brief flash, etc.)
• History is bounded at 100 entries (existing behaviour, just verify)

Files to modify

• public/design-mode-agent.js — add keydown listener in iframe
• components/workspace/preview-panel.tsx — add listener on the wrapper
• components/workspace/workspace-header.tsx — add hint icon in toolbar

Technical Notes

• The shortcut should NOT trigger when an <input> or <textarea> has focus (so it doesn't break form editing)
• Use e.metaKey (macOS) or e.ctrlKey (others) to detect the modifier
• The handler should call parent.postMessage({ type: 'design-undo' | 'design-redo' }, '*') from inside the iframe
• The parent (preview-panel.tsx) listens for the message and calls the API

Related

• Issue feat: Undo/Redo history for Design Mode #52 (closed)
• lib/workspace/design-history.ts (already implemented)
• app/api/workspace/design-history/route.ts (already implemented)

[Delqhi]

Implemented and merged in #64 (commit 027b464).

Added ⌘Z / ⌘⇧Z keyboard shortcuts in Design Mode:

• public/design-mode-agent.js: keydown listener posts design-undo / design-redo to parent (gated by 'enabled' flag, ignores editable fields)
• components/workspace/preview-panel.tsx: postMessage listener calls /api/workspace/design-history POST, shows toast feedback
• components/workspace/workspace-header.tsx: ⌘Z / ⌘⇧Z hint icon visible in Design tab