feat(theming): in-UI theme editor (2.10.0)

Recolour the whole UI from Settings — 12 palette colour pickers with
live preview, presets (Ember/Frost/Verdant/Amethyst), and reset to the
default Blackflame. Overrides persist in app_settings (key 'theme') via
a hex-validated /api/theme route and apply to :root on boot.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "void-server",
-  "version": "2.9.0",
+  "version": "2.10.0",
   "type": "module",
   "private": true,
   "scripts": {