Add NavMenu settings/presets UI and profile HTTP routes (ticket 011)

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

webapp/src/index.css

@@ -15,3 +15,17 @@ body {
   @apply bg-neutral-950 text-neutral-200;
   font: 14px/1.5 ui-monospace, Consolas, monospace;
 }
+
+/*
+ * Ticket 011: colour-scheme picker support. Most of the app's Tailwind
+ * classes are hardcoded to a dark palette (e.g. `bg-neutral-950` above)
+ * rather than using `dark:`-variant classes, so there isn't a full light
+ * theme to switch to yet. This override (keyed off the `light` class
+ * `useSettingsPanel` toggles on `<html>`) gives "Light" a real, if minimal,
+ * visible effect on the base page colours; a proper light theme would need
+ * the rest of the app's components reworked to use theme-aware classes.
+ */
+html.light body {
+  background-color: #f5f5f4;
+  color: #1c1917;
+}