PC controls: keyboard pan/zoom, Tab cycle, Escape stack, right-click deselect

Adds full PC keyboard+mouse support on top of existing touch controls. Touch
paths untouched. All input goes through named actions in project.godot.

Bindings:
- WASD / arrows: camera pan (speed scales with zoom)
- = / -: keyboard zoom in/out
- C / Home: center on selected pawn
- Tab / Shift+Tab: cycle through pawns (pans camera to selection)
- B / L / P / ,: toggle BuildDrawer / AlertsLog / WorkPriorityMatrix / Settings
- Escape: cancel active designation tool > close topmost panel > deselect pawn
- Right-click: cancel active tool or deselect pawn (RTS convention)
- F: speed_cycle (action restored; handler still TODO)
- pawn_prev action removed; Shift+Tab read via event.shift_pressed inline

Escape priority enforced by Designation._input running before _unhandled_input
plus each panel consuming its own cancel action when visible.

Also fixes a pre-existing pre-Phase-17 bug: WorkPriorityMatrix, AlertsLog,
StorytellerModal, LoadMenu, and SettingsMenu had MOUSE_FILTER_STOP Controls
(Backdrop / Dim) that remained input-active when the panel was "closed" —
their open/close paths only toggled _root.visible / _panel.visible, never
CanvasLayer.visible. World mouse events (right-click deselect, left-click
pawn-select) were silently eaten. Now each _set_visible / open / close
toggles self.visible (the CanvasLayer) so input dispatch shuts off properly.

Verified end-to-end via MCP runtime: WASD pan, zoom keys, Tab+Shift+Tab
cycle, B-open + Escape-close, right-click deselect, left-click pawn-select
all working in sequence with no input bleed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scenes/ui/load_menu.gd

@@ -43,6 +43,19 @@ func _exit_tree() -> void:
 	pass
 
 
+func _unhandled_input(event: InputEvent) -> void:
+	# _set_visible drives _panel.visible, not CanvasLayer.visible — use _panel as the gate.
+	if _panel == null or not _panel.visible:
+		return
+	if event.is_action_pressed("cancel"):
+		# If the version-mismatch sub-dialog is open, dismiss it first.
+		if _warn_panel != null and _warn_panel.visible:
+			_on_warn_cancel()
+		else:
+			_close()
+		get_viewport().set_input_as_handled()
+
+
 # ── public API ────────────────────────────────────────────────────────────────
 
 func open() -> void:
@@ -246,6 +259,10 @@ func _close() -> void:
 
 
 func _set_visible(v: bool) -> void:
+	# Toggle CanvasLayer.visible so the dim Control (MOUSE_FILTER_STOP) does not
+	# eat _unhandled_input mouse events for the world below when modal is hidden.
+	# (The warning sub-dialog visibility is managed separately and lives inside.)
+	visible = v or (_warn_panel != null and _warn_panel.visible)
 	if _dim != null:
 		_dim.visible = v
 	if _panel != null: