perf: desktop DOM element reuse + manifest ETag caching

[MCERQUA]

Problem

1. Every manifest poll (30s) destroys and recreates ALL desktop icon DOM nodes via innerHTML. At 200+ pages this causes visible UI stutter (~500ms CPU time per cycle).

2. Full manifest JSON is re-fetched and re-parsed every 30 seconds even when nothing changed. No conditional requests (ETag/If-None-Match).

Fix

1. Reuse existing DOM elements — diff against current DOM, only add/remove/update changed icons.
2. Add ETag header support to manifest API and use conditional fetch in the client.

Scaling Impact

Pages	Current (innerHTML rebuild)	After (element reuse)
50	Fine	Fine
200	~500ms stutter every 30s	< 10ms diff
500	~1-2s stutter, unusable	~50ms diff

Reference

• Refactor Plan — Phase 3