feat: add playable Blokus web UI with replay system

- Add blokus_ui package: Flask web app for human-vs-bot Blokus
- GameSession: wraps BlokusGame for human play, records move history, serializes
- RunStore: file-based JSON replay storage with save/load/list/delete
- Flask app: routes for new game, play, save, browse runs, replay
- Templates: index (new game + run browser), game (playable board), replay (step controls)
- Static: CSS (dark theme, colored cells, placement overlays) + JS (click handling, API calls)
- Add blokus_ui tests (19 tests for GameSession and RunStore)
- Update pyproject.toml with flask dependency and blokus-ui entry point
- Fix: remove unreachable dead code in BlokusGame.valid_move
- All 102 tests pass, ruff lint clean

src/blokus_ui/__init__.py

@@ -0,0 +1,10 @@
+from blokus_ui.session import GameSession
+from blokus_ui.store import RunStore
+
+__version__ = "0.1.0"
+
+__all__ = [
+    "GameSession",
+    "RunStore",
+    "__version__",
+]