Support multiple named, saved custom themes per user

Replaces the single custom_theme_colors blob (one palette per user) with
a proper CustomTheme table -- users can now save, name, and switch
between as many custom palettes as they like, not just one.

Data model: users.active_custom_theme_id references whichever saved
CustomTheme (if any) is currently active; theme='custom' + that id
together determine what's rendered. The migration data-migrates any
already-saved single palette into a named CustomTheme row on upgrade,
and best-effort backfills the active one back into the old column shape
on downgrade.

New endpoints under /api/custom-themes: list, create, rename/recolor,
delete (falls back the user to a preset if the deleted theme was
active, so the two theme columns can never disagree), and activate.
UserRead.active_custom_theme is only populated when theme == 'custom'
even though the DB deliberately keeps the id set while a preset is
active, so switching to a preset and back doesn't lose the saved
palette.

ProfileModal now lists saved themes as swatches (click to activate,
pencil to edit -- active or not, trash to delete with a confirm), plus
a "+ New" button that creates, activates, and opens the editor for a
fresh theme immediately.

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

backend/app/services/auth_service.py

@@ -1,6 +1,7 @@
 from sqlalchemy import or_, select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.models import User
 from app.schemas.user import UserCreate
@@ -40,9 +41,9 @@ async def authenticate_user(
     db: AsyncSession, username_or_email: str, password: str
 ) -> User:
     result = await db.execute(
-        select(User).where(
-            or_(User.username == username_or_email, User.email == username_or_email)
-        )
+        select(User)
+        .where(or_(User.username == username_or_email, User.email == username_or_email))
+        .options(selectinload(User.active_custom_theme))
     )
     user = result.scalar_one_or_none()
     if user is None or not verify_password(password, user.password_hash):