polish: discovery hardening per code review (review round 2 minors)

- Fail fast with clear RuntimeError when themes/ directory is missing
- Document underscore-prefix skip rule
- Tests: broken-manifest/mismatch/no-manifest skip case, missing-dir fail-fast
- tests/test_theme_pro_v3.py: drop fourth hardcoded list, import from core

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

app/core.py

@@ -13,8 +13,12 @@ ROOT = Path(__file__).resolve().parent.parent
 
 def discover_themes():
     """Discover built-in themes from themes/*/manifest.json (single source of truth)."""
+    themes_dir = ROOT / 'themes'
+    if not themes_dir.is_dir():
+        raise RuntimeError(f'themes directory missing: {themes_dir}')
     found = []
-    for entry in sorted((ROOT / 'themes').iterdir()):
+    for entry in sorted(themes_dir.iterdir()):
+        # Underscore prefix marks non-theme assets (e.g. _professional_components.html).
         if not entry.is_dir() or entry.name.startswith('_'):
             continue
         try: