chore: add CI, lint config, login rate limit and split large modules

- Rate-limit failed logins (8 per 15 min per IP or username, 429 + Retry-After)
- Add ruff/mypy config and GitHub Actions workflow (ruff, pytest, tsc, tests, build)
- Apply ruff autofixes (import order, pyupgrade)
- Extract rpy_helpers.py, engine-util.ts, TranslationParts.tsx and
  translation-types.ts from oversized modules
- Add dependency-free React UI tests
- Refresh README and ARCHITECTURE (Naninovel/Tomie, status and plans)

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

backend/app/converter/unity_bundle.py

@@ -3,8 +3,8 @@ from pathlib import Path
 
 
 def unpack(source: Path, target: Path) -> list[Path]:
-    from UnityPy.streams import EndianBinaryReader
     from UnityPy.helpers.CompressionHelper import DECOMPRESSION_MAP
+    from UnityPy.streams import EndianBinaryReader
 
     target.mkdir(parents=True, exist_ok=True)
     with source.open("rb") as stream: