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/translation.py

@@ -384,7 +384,7 @@ def export_po(manifest: dict[str, Any], lang: str, game_title: str) -> str:
                 f"msgstr {_po_quote(e.get('t', ''))}", ""]
     for src, usage in idx.strings.items():
         e = strings.get(src) or {}
-        out += [f"#. {usage}", f'msgctxt "string"', f"msgid {_po_quote(src)}",
+        out += [f"#. {usage}", 'msgctxt "string"', f"msgid {_po_quote(src)}",
                 f"msgstr {_po_quote(e.get('t', ''))}", ""]
     return "\n".join(out)