bug: fetchManifest/saveState race condition loses icon positions

[MCERQUA]

Problem

saveState() is debounced at 500ms. fetchManifest() polls every 30s and rebuilds desktop items. If user drags an icon and the manifest poll fires before the debounced save completes, the icon position is lost.

Fix

Add a dirty flag. Skip manifest rebuild if state has unsaved local changes.

Reference

• Refactor Plan — Phase 1.2
• Lines: 966, 884 in default-pages/desktop.html