grid: j/k move the cursor down/up one row; stop blanking thumbnails on nav

_on_nav_changed rebuilt every cell on every navigator "changed" signal,
so a single h/l/j/k press destroyed and recreated all 613 GtkPicture
cells (each re-requesting its thumbnail) and the grid flashed blank.
Now it only does a full ggaze_grid_refresh when the file count actually
changed (rescan/trash/move); a mere current/mark change just refreshes
mark badges in place and re-selects the cursor.

Add j/k row navigation: ggaze_grid_move_cursor finds the cell in the
adjacent row closest to the current column via gtk_widget_compute_bounds
and sets navigator.current to it (so the header / large-view preview
track the move and the light nav-changed path re-selects it). h/l stay
linear via the existing global win.next/prev. Also document j/k in the
? shortcuts overlay.

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