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/password_service.py

@@ -3,6 +3,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.models import PasswordReset, User
 from app.security import hash_password, hash_token, verify_password
@@ -70,7 +71,9 @@ async def validate_reset_token(db: AsyncSession, token: str) -> None:
 
 async def complete_password_reset(db: AsyncSession, token: str, new_password: str) -> User:
     reset = await _get_valid_reset(db, token)
-    user = await db.get(User, reset.user_id)
+    user = await db.get(
+        User, reset.user_id, options=[selectinload(User.active_custom_theme)]
+    )
     user.password_hash = hash_password(new_password)
     reset.used = True
     await db.commit()