Adds thin HTTP routes for profile CRUD and colour-scheme updates
(GET/PATCH /api/settings, GET/POST /api/settings/profiles,
DELETE /api/settings/profiles/:name, POST /api/settings/profiles/:name/activate)
following the existing CSS-route pattern in api.rs, with activate hot-swapping
the running engine's declared params/metrics so switching profiles needs no
restart. Builds out the NavMenu frontend with Settings (colour scheme picker,
CSS import/list/delete) and Presets (list/create/switch/delete profiles with
declaration counts) panels, and a useSettingsPanel hook that applies the
active colour scheme and imported CSS to the document live.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
