feat: Implement interactive font selection by scanning the fonts/ directory for .otf, .ttf, and .ttc files, adding new fonts and updating documentation.

engine/render.py

@@ -57,6 +57,10 @@ _FONT_CACHE = {}
 def font():
     """Lazy-load the primary OTF font (path + face index aware)."""
     global _FONT_OBJ, _FONT_OBJ_KEY
+    if not config.FONT_PATH:
+        raise FileNotFoundError(
+            f"No primary font selected. Add .otf/.ttf/.ttc files to {config.FONT_DIR}."
+        )
     key = (config.FONT_PATH, config.FONT_INDEX, config.FONT_SZ)
     if _FONT_OBJ is None or _FONT_OBJ_KEY != key:
         _FONT_OBJ = ImageFont.truetype(