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/schemas/user.py

@@ -1,5 +1,6 @@
 import uuid
 from datetime import datetime
+from typing import Literal
 
 from pydantic import BaseModel, ConfigDict, EmailStr, Field
 
@@ -19,12 +20,19 @@ class UserRead(BaseModel):
     is_bot: bool
     is_site_admin: bool
     display_name: str | None
+    theme: str | None
     avatar_filename: str | None
     created_at: datetime
 
 
 class ProfileUpdate(BaseModel):
+    # Both fields are independently optional-and-settable -- the router
+    # only applies keys actually present in the request body
+    # (model_dump(exclude_unset=True)), so a call that only wants to change
+    # the theme doesn't clobber display_name back to None, and vice versa.
     display_name: str | None = Field(default=None, max_length=50)
+    # Kept in sync with frontend/src/styles/themes.css's theme blocks.
+    theme: Literal["dark", "light", "midnight", "sunset"] | None = Field(default=None)
 
 
 class UserDirectoryRead(BaseModel):