feat: single-source theme discovery replaces three hardcoded whitelists

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

app/theme_store.py

@@ -8,10 +8,10 @@ from __future__ import annotations
 import io, json, re, zipfile, shutil
 from pathlib import Path
 from . import db
+from .core import THEMES as BUILTIN
 
 BASE=Path(__file__).resolve().parent.parent/'themes'
 THEME_ROOT=db.DATA/'theme_packages'
-BUILTIN=('automotive','industrial','corporate','minimal')
 THEME_ID=re.compile(r'^[a-z][a-z0-9-]{2,39}$')
 VERSION=re.compile(r'^\d+\.\d+\.\d+$')