Add UI themes (Dark, Light, Midnight, Sunset)

Four preset color themes, switchable from Profile settings with instant
preview and server-side persistence (User.theme, applied via a data-theme
attribute the CSS custom-property overrides in themes.css key off). Dark
stays the existing DarkSingularity palette and default. Light is a genuine
new light-mode design; Midnight is a higher-contrast OLED-friendly dark
variant; Sunset swaps in a warm amber/coral accent family.

Custom theme building (pick-your-own-colors) is out of scope for this
pass -- presets only.

Also: fixed the PATCH /api/auth/me handler to only apply fields actually
present in the request body. It previously always overwrote display_name
unconditionally, which happened to be harmless when it was the only
field on ProfileUpdate but would have silently cleared it on any
theme-only update. And switched two hardcoded hex colors
(.btn-primary:hover, .role-badge-admin) to token-derived color-mix()
values so they adapt across themes instead of staying fixed to the
original cyan/violet palette.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/context/AuthContext.tsx

@@ -20,6 +20,10 @@ export function AuthProvider({ children }: { children: ReactNode }) {
   const [loading, setLoading] = useState(true)
   const [offline, setOffline] = useState(false)
 
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', user?.theme ?? 'dark')
+  }, [user?.theme])
+
   useEffect(() => {
     authApi
       .me()