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/locales.py

@@ -8,11 +8,11 @@
 
 import hashlib
 import re
-from uuid import uuid4
 from collections import Counter, defaultdict
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any
+from uuid import uuid4
 
 from app.converter.rpy_lexer import Cursor, Line, ParseError, parse_tree
 from app.converter.source import read_source