Original PR: #2676 by @lucasrc
Adds POST /api/skills/toggle endpoint that flips skills.disabled in
config.yaml, and a UI toggle in the Skills panel that shows all skills
(including disabled ones) with a per-skill on/off control.
- Backend: new endpoint validates skill exists in filesystem before
toggling. Read-modify-write wrapped in _cfg_lock for thread safety.
Writes through to platform_disabled.webui when present.
- Frontend: each skill-item now has a toggle switch; disabled skills
appear muted but still listed (previously they were filtered out).
- i18n: new toggle keys translated across all 9 non-English locales.
- Tests: round-trip test for disabled list normalization + toggle
endpoint behavior.
Squash-merged from contributor's branch (19 commits + 1 merge commit)
onto current master via the cherry-pick-stale-contributor-prs procedure.
