feat: mousearea, datetime dates, dir(), renpy.image_size and style.rebuild for Ren'Py games

Screens get an invisible mousearea that reports the pointer entering and leaving without taking
clicks. datetime.date/datetime/timedelta support construction and date arithmetic. dir() lists the
game's variables, renpy.style.rebuild() is a no-op, and renpy.image_size answers only for pictures
the browser already holds (otherwise an IOError the game can catch).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/app/converter/screens.py

@@ -31,7 +31,7 @@ Node = dict[str, Any]
 OPENERS = {"Show", "ShowTransient", "ToggleScreen", "Hide", "show_screen", "call_screen", "hide_screen"}
 
 CONTAINERS = {"fixed", "frame", "window", "vbox", "hbox", "grid", "vpgrid", "viewport", "button", "side", "nearrect"}
-LEAVES = {"text", "label", "textbutton", "imagebutton", "add", "image", "null", "bar", "vbar"}
+LEAVES = {"text", "label", "textbutton", "imagebutton", "add", "image", "null", "bar", "vbar", "mousearea"}
 POSITIONAL = {"side": 1, "text": 1, "label": 1, "textbutton": 1, "add": 1, "image": 1, "grid": 2}
 ATL_WORDS = {"pause", "repeat", "parallel", "choice", "block", "contains", "function", "time"}