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

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+extends SceneTree
+## Verifica la scorciatoia Alt+F (finestra <-> schermo intero):
+##  - l'azione "toggle_fullscreen" esiste nella mappa dei comandi ed e' Alt+F;
+##  - ogni pressione alterna lo stato, anche a partita avviata e a cursore catturato;
+##  - l'evento viene marcato come gestito (il tasto non arriva al gioco);
+##  - Alt+F senza Alt (o F da solo) NON cambia nulla.
+## Avviare con:  --headless --path . --script res://tools/test_fullscreen_shortcut.gd
+## Con un display reale si usa:  --rendering-method gl_compatibility
+const ACTIONS := ["toggle_fullscreen"]
+
+var world: Node3D
+var failures := 0
+var checks := 0
+
+func _initialize() -> void:
+	call_deferred("run_test")
+
+func check(label: String, condition: bool, detail: String = "") -> void:
+	checks += 1
+	if condition:
+		print("PASS: " + label)
+	else:
+		failures += 1
+		print("FAIL: %s %s" % [label, detail])
+
+func run_test() -> void:
+	# --- mappa dei comandi ---
+	check("l'azione toggle_fullscreen esiste nella mappa", InputMap.has_action("toggle_fullscreen"))
+	var alt_f := false
+	for event in InputMap.action_get_events("toggle_fullscreen"):
+		if event is InputEventKey and event.alt_pressed \
+		and (event.physical_keycode == KEY_F or event.keycode == KEY_F):
+			alt_f = true
+	check("l'azione corrisponde ad Alt+F", alt_f, str(InputMap.action_get_events("toggle_fullscreen")))
+
+	world = load("res://scenes/main.tscn").instantiate()
+	root.add_child(world)
+	await process_frame
+	await process_frame
+
+	var start_state: bool = world.fullscreen
+	_press_f(true)
+	await process_frame
+	check("Alt+F capovolge lo stato dello schermo intero",
+		world.fullscreen != start_state, "prima=%s dopo=%s" % [str(start_state), str(world.fullscreen)])
+	_press_f(true)
+	await process_frame
+	check("una seconda pressione riporta allo stato iniziale",
+		world.fullscreen == start_state, "dopo=%s atteso=%s" % [str(world.fullscreen), str(start_state)])
+
+	# F senza Alt non deve cambiare nulla
+	var before: bool = world.fullscreen
+	_press_f(false)
+	await process_frame
+	check("F da solo (senza Alt) non cambia lo schermo", world.fullscreen == before,
+		"prima=%s dopo=%s" % [str(before), str(world.fullscreen)])
+
+	# --- in partita, a cursore catturato ---
+	var screen = world.start_screen
+	var gioca: Button = null
+	if screen != null:
+		for button in screen.find_children("*", "Button", true, false):
+			if button.text == "GIOCA":
+				gioca = button
+	if screen != null and screen.get("name_field") != null:
+		screen.name_field.text = "AltF"
+	if gioca != null:
+		gioca.pressed.emit()
+	await process_frame
+	await process_frame
+	check("la partita e' iniziata (il cursore e' catturato o headless)",
+		world.player.playing == true, "playing=%s" % str(world.player.playing))
+	var during_play: bool = world.fullscreen
+	_press_f(true)
+	await process_frame
+	check("Alt+F funziona anche in partita, a cursore catturato",
+		world.fullscreen != during_play, "prima=%s dopo=%s" % [str(during_play), str(world.fullscreen)])
+	check("Alt+F non interrompe la partita ne' libera il cursore",
+		world.player.playing == true and world.player.death_started == false,
+		"playing=%s morte=%s" % [str(world.player.playing), str(world.player.death_started)])
+
+	print("%d controlli superati, %d falliti" % [checks - failures, failures])
+	quit(1 if failures > 0 else 0)
+
+## Pressione + rilascio di F, con o senza Alt: stesso percorso di un tasto reale.
+func _press_f(alt: bool) -> void:
+	for pressed in [true, false]:
+		var event := InputEventKey.new()
+		event.physical_keycode = KEY_F
+		event.keycode = KEY_F
+		event.alt_pressed = alt
+		event.pressed = pressed
+		Input.parse_input_event(event)
+	Input.flush_buffered_events()

tools/test_fullscreen_shortcut.gd.uid

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+uid://bdaw8q5cc5owc