grid: mark multi-selection (v / Ctrl+a / Esc) and a ? shortcuts overlay

The navigator already had a mark API and the grid rendered mark badges,
but nothing could set marks — so multi-file ops (D, Ctrl+c, m) only ever
acted on the single current image. Wire up marking in the UI:

- v toggles a mark on the highlighted grid cell (or the current image in
  large view). navigator_toggle_mark does not emit "changed", so the
  cell's badge is updated in place via ggaze_grid_update_mark_badge
  (no reflow / re-decode).
- Ctrl+a marks all; Esc now clears marks first (contextual), then keeps
  its existing back/quit behavior.
- The window title appends "N marked" so multi-selection is visible.
- A GtkCssProvider styles .ggaze-marked (accent border + tint) — the
  class was set but had no styling, so badges were invisible.

Also add a ? -> win.shortcuts action that presents a GtkShortcutsWindow
(grouped: navigation, view, selection, files, zoom) built from an inline
GtkBuilder UI string, transient to the window.

tests/test_shortcut.c full-table test updated for the three new bindings
(v, Ctrl+a, ?) -> 23 rows, 19 actions.

1 files changed, 8 insertions, 7 deletions

diff --git a/tests/test_shortcut.c b/tests/test_shortcut.c
index db68e7b..e57d361 100644
--- a/tests/test_shortcut.c
+++ b/tests/test_shortcut.c
@@ -328,10 +328,11 @@ test_shortcut_keypath_toggle_and_back(void) {
 static void
 test_shortcut_full_table_registered(void) {
    static const char *ACTIONS[] = {
-      "win.prev",       "win.next",        "win.first",   "win.last",
-      "win.open",       "win.quit",        "win.trash",   "win.delete",
-      "win.undo",       "win.toggle-view", "win.zoom-in", "win.zoom-out",
-      "win.fullscreen", "win.slideshow",   "win.info",    "win.back",
+      "win.prev",      "win.next",        "win.first",    "win.last",
+      "win.open",      "win.quit",        "win.trash",    "win.delete",
+      "win.undo",      "win.toggle-view", "win.mark",     "win.mark-all",
+      "win.shortcuts", "win.zoom-in",     "win.zoom-out", "win.fullscreen",
+      "win.slideshow", "win.info",        "win.back",
    };
    GgazeWindow           *p_win = new_window();
    GtkShortcutController *p_sc  = find_shortcut_controller(GTK_WIDGET(p_win));
@@ -341,9 +342,9 @@ test_shortcut_full_table_registered(void) {
       g_assert_nonnull(p_s);
       g_object_unref(p_s);
    }
-   /* The SHORTCUTS[] table has 20 rows (some actions appear twice, e.g.
-    * win.prev for h and Left). The controller must carry all 20. */
-   g_assert_cmpint(g_list_model_get_n_items(G_LIST_MODEL(p_sc)), ==, 20);
+   /* The SHORTCUTS[] table has 23 rows now (some actions appear twice, e.g.
+    * win.prev for h and Left; win.zoom-in for plus and equal). */
+   g_assert_cmpint(g_list_model_get_n_items(G_LIST_MODEL(p_sc)), ==, 23);
    g_object_unref(p_sc);
    g_object_unref(p_win);
    drain_main(200);