fix: eliminate async safety violations and migrate to pathlib (#697)

## ℹ️ Description
Eliminate all blocking I/O operations in async contexts and modernize
file path handling by migrating from os.path to pathlib.Path.

- Link to the related issue(s): #692 
- Get rid of the TODO in pyproject.toml
- The added debug logging will ease the troubleshooting for path related
issues.

## 📋 Changes Summary

- Enable ASYNC210, ASYNC230, ASYNC240, ASYNC250 Ruff rules
- Wrap blocking urllib.request.urlopen() in run_in_executor
- Wrap blocking file operations (open, write) in run_in_executor
- Replace blocking os.path calls with async helpers using
run_in_executor
- Replace blocking input() with await ainput()
- Migrate extract.py from os.path to pathlib.Path
- Use Path() constructor and / operator for path joining
- Use Path.mkdir(), Path.rename() in executor instead of os functions
- Create mockable _path_exists() and _path_is_dir() helpers
- Add debug logging for all file system operations

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [X] 🐞 Bug fix (non-breaking change which fixes an issue)
- [ ] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)


## ✅ Checklist
Before requesting a review, confirm the following:
- [X] I have reviewed my changes to ensure they meet the project's
standards.
- [X] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [X] I have formatted the code (`pdm run format`).
- [X] I have verified that linting passes (`pdm run lint`).
- [X] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
  * Made user prompt non‑blocking to improve responsiveness.
* Converted filesystem/path handling and prefs I/O to async‑friendly
operations; moved blocking network and file work to background tasks.
* Added async file/path helpers and async port‑check before browser
connections.

* **Tests**
* Expanded unit tests for path helpers, image download success/failure,
prefs writing, and directory creation/renaming workflows.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/kleinanzeigen_bot/resources/translations.de.yaml

@@ -173,7 +173,10 @@ kleinanzeigen_bot/__init__.py:
 kleinanzeigen_bot/extract.py:
 #################################################
   download_ad:
-    "Created ads directory at ./%s.": "Verzeichnis für Anzeigen erstellt unter ./%s."
+    "Ensured ads directory exists at ./%s.": "Verzeichnis [%s] für Anzeige vorhanden."
+
+  _download_and_save_image_sync:
+    "Failed to download image %s: %s": "Fehler beim Herunterladen des Bildes %s: %s"
 
   _download_images_from_ad_page:
     "Found %s.": "%s gefunden."