Sync CE: refine slash launcher and dark mode surfaces

[Luhaozhu]

Summary

• keep the slash launcher focused on installed plugins and enabled skills, while conversation modes remain in the at-sign launcher
• unify dark-mode sidebar, welcome hero, and recommendation-banner surfaces to remove visible background blocks
• align the CE desktop version metadata with 0.5.15

Validation

• python scripts/build_ce.py --import-check --pytest-check --frontend-check with Node.js 20.20.2
• CE branding, prohibited-artifact, binary-asset, and LICENSE gates passed
• CE import/OpenAPI/ORM checks, release regression tests, and frontend production build passed