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.

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