fix(webui): restore session drag and harden frontend loading

[chengyongru]

Summary

• restore dragging inactive standalone sessions and workbench panes into the composer as session mentions; tab grouping chrome remains non-draggable
• make service-worker pruning follow Vite's complete build manifest so current lazy chunks are retained
• lazy-load locale resources and heavy Feishu/Weixin panels, reducing the entry bundle from about 447 KB to 208 KB gzip
• keep browser zoom enabled, make collapsed reasoning content inert, and remove the unused mention wrapper

This is a follow-up to #5322, which kept the composer drop consumer but removed the sidebar session drag producer during the tabbed workbench rewrite.

Verification

• focused WebUI tests: 156 passed
• bun run lint
• bun run build
• real gateway + Playwright: standalone session and workbench pane drag-to-composer both create session mentions
• controlled service-worker reload: asset manifest cached, console 0 errors / 0 warnings