Add a main menu scene with Start/Hard Mode/Music options

The game now boots into a menu (Start Game, Hard Mode toggle, Music
toggle) navigated with Up/Down and A/Start, instead of dropping
straight into a match. B returns to the menu from a paused or
game-over state.

source/game_input.cpp

@@ -29,6 +29,43 @@ bool handle_input_frame(InputContext &ctx) {
         }
     }
 
+    if (ctx.state->scene == Scene::Menu) {
+        if ((keys != 0U) && ((keys & (KEY_UP | KEY_DOWN)) != 0U)) {
+            const int delta = ((keys & KEY_UP) != 0U) ? -1 : 1;
+            ctx.state->menuSelection = (ctx.state->menuSelection + delta + MENU_ITEM_COUNT) % MENU_ITEM_COUNT;
+            draw_menu(*ctx.state, *ctx.renderAssets, audio_is_music_enabled(*ctx.audio));
+        }
+
+        if ((keys != 0U) && (((keys & KEY_A) != 0U) || ((keys & KEY_START) != 0U))) {
+            switch (ctx.state->menuSelection) {
+            case 0:
+                ctx.state->scene = Scene::Playing;
+                ctx.init_game();
+                draw_game(*ctx.state, *ctx.renderAssets);
+                break;
+            case 1:
+                ctx.state->hardMode = !ctx.state->hardMode;
+                draw_menu(*ctx.state, *ctx.renderAssets, audio_is_music_enabled(*ctx.audio));
+                break;
+            case 2:
+                audio_toggle_music(*ctx.audio);
+                draw_menu(*ctx.state, *ctx.renderAssets, audio_is_music_enabled(*ctx.audio));
+                break;
+            default:
+                break;
+            }
+        }
+
+        return true;
+    }
+
+    if ((ctx.state->paused || ctx.state->gameOver) && (keys != 0U) && ((keys & KEY_B) != 0U)) {
+        ctx.state->scene = Scene::Menu;
+        ctx.state->paused = false;
+        draw_menu(*ctx.state, *ctx.renderAssets, audio_is_music_enabled(*ctx.audio));
+        return true;
+    }
+
     if (!ctx.state->paused && !ctx.state->gameOver) {
         const bool cursed = ctx.state->curseTicksRemaining > 0;