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>

backend/app/routers/auth.py

@@ -74,8 +74,15 @@ async def update_profile(
     current_user: User = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ) -> User:
-    display_name = data.display_name.strip() if data.display_name else None
-    current_user.display_name = display_name or None
+    # Only apply fields actually present in the request body -- a call that
+    # only wants to change the theme must not clobber display_name back to
+    # None, and vice versa.
+    updates = data.model_dump(exclude_unset=True)
+    if "display_name" in updates:
+        display_name = updates["display_name"].strip() if updates["display_name"] else None
+        current_user.display_name = display_name or None
+    if "theme" in updates:
+        current_user.theme = updates["theme"]
     await db.commit()
     await db.refresh(current_user)
     return current_user