Add keyboard play: arrows/WASD/hjkl focus, Enter/Space flips

Focus ring appears on the first key press and hides when the mouse moves.
Enter also triggers Play Again on the win panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/memory_test.odin

@@ -270,3 +270,23 @@ card_pulse_counts_down :: proc(t: ^testing.T) {
     card.update(&c, 1)
     testing.expect_value(t, c.pulse_t, 0)
 }
+
+@(test)
+keyboard_focus_stays_inside_grid :: proc(t: ^testing.T) {
+    memory.deal(.HIRAGANA, .NORMAL) // 4 rows x 6 columns
+    testing.expect_value(t, memory.focus, 0)
+
+    memory.move_focus(-1, -1) // already top-left
+    testing.expect_value(t, memory.focus, 0)
+    memory.move_focus(1, 0)
+    testing.expect_value(t, memory.focus, 1)
+    memory.move_focus(0, 1)
+    testing.expect_value(t, memory.focus, 7)
+    memory.move_focus(100, 100) // clamps to bottom-right
+    testing.expect_value(t, memory.focus, memory.card_count - 1)
+
+    memory.deal(.HIRAGANA, .HARD) // dealing resets focus
+    testing.expect_value(t, memory.focus, 0)
+    memory.move_focus(0, 3)
+    testing.expect_value(t, memory.focus, 24) // 4 rows x 8 columns
+}