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, 3 insertions, 0 deletions

diff --git a/src/shortcuts.c b/src/shortcuts.c
index 0c737bd..8ea978d 100644
--- a/src/shortcuts.c
+++ b/src/shortcuts.c
@@ -33,6 +33,9 @@ static const ShortcutEntry SHORTCUTS[] = {
    {GDK_KEY_D, GDK_SHIFT_MASK, "win.delete"},
    {GDK_KEY_u, 0, "win.undo"},
    {GDK_KEY_t, 0, "win.toggle-view"},
+   {GDK_KEY_v, 0, "win.mark"},
+   {GDK_KEY_a, GDK_CONTROL_MASK, "win.mark-all"},
+   {GDK_KEY_question, 0, "win.shortcuts"},
    {GDK_KEY_plus, 0, "win.zoom-in"},
    {GDK_KEY_equal, 0, "win.zoom-in"},
    {GDK_KEY_minus, 0, "win.zoom-out"},