Add Alt+F to toggle fullscreen and window mode

New remappable action `toggle_fullscreen` (Alt+F) handled in main.gd::_input(),
so it works in the menu, during a match with the pointer captured and in the boss
sandbox; the event is marked handled so the key does not reach the rest of the game.

toggle_fullscreen() switches between WINDOW_MODE_FULLSCREEN and WINDOW_MODE_WINDOWED;
the state lives in main.fullscreen (initialised from the real window mode) instead of
querying the driver on every press, so the toggle is deterministic and testable. On
Web it is a no-op because fullscreen belongs to the page.

Desktop HUD hint updated. New tools/test_fullscreen_shortcut.gd: 8 checks (action and
key mapping, alternation, bare F inert, works mid-match without freeing the cursor).
Verified on a real display: window_get_mode 0 -> 3 -> 0.

tools/test_fullscreen_shortcut.gd.uid

@@ -0,0 +1 @@
+uid://bdaw8q5cc5owc