feat: add configurable speed and expanded grid presets for snake

- Add per-game speed setting with presets (Slow/Normal/Fast/Insane)
- Add speed selector UI in snake lobby
- Expand grid presets with Tiny (15x15) and XL (50x30)
- Auto-calculate cell size based on grid dimensions
- Preserve speed setting in rematch games

db/migrations/005_add_snake_speed.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE games ADD COLUMN snake_speed INTEGER NOT NULL DEFAULT 7;
+
+-- +goose Down
+ALTER TABLE games DROP COLUMN snake_speed;