shortcuts: GLOBAL scope so t/Esc reach window actions (2u0)

The GtkShortcutController on the window used MANAGED scope, so the
viewer's own GtkEventControllerKey consumed key events before the
window-level win.* shortcuts were consulted — t (toggle-view) and Esc
(back) never fired. Switch to GLOBAL scope, which registers the
shortcuts with the toplevel's global shortcut manager consulted before
child key controllers. Comment notes the future-text-entry caveat.

New integration test tests/test_shortcut.c (3 subtests):
- controller_scope: asserts the window's shortcut controller is GLOBAL
  (and is ours, not GtkApplicationWindow's mnemonic controller).
- keypath_toggle_and_back: verifies the t→win.toggle-view and
  Escape→win.back bindings (bidirectional keyval/action check) and
  dispatches them via gtk_shortcut_action_activate (the controller's own
  dispatch primitive), asserting the stack flips large↔grid. GTK 4.22
  exposes no public GdkKeyEvent synthesis API, so this is the most
  faithful exercise of the keyboard shortcut path available.
- full_table_registered: asserts all 16 win.* actions and all 20
  shortcut rows are registered, guarding against table regressions.

Notes a pre-existing, out-of-scope keybinding conflict: GTK4 normalizes
GDK_KEY_G/0 to GDK_KEY_g/0 in GtkKeyvalTrigger, so win.last (G) collides
with win.first (g); left unfixed per task's 'preserve existing behavior'.

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