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.

project.godot

@@ -83,6 +83,12 @@ pause={
 ]
 }
 
+toggle_fullscreen={
+"deadzone": 0.5,
+"events": [Object(InputEventKey, "resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":true,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":70,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+
 grenade={
 "deadzone": 0.5,
 "events": [Object(InputEventMouseButton, "button_index":2)]