feat: integrate XDG paths into bot core (#776)

## ℹ️ Description
Wire XDG path resolution into main bot components.

- Link to the related issue(s): N/A (new feature)
- Integrates installation mode detection into bot core

## 📋 Changes Summary

- Added `finalize_installation_mode()` method for mode detection
- UpdateChecker, AdExtractor now respect installation mode
- Dynamic browser profile defaults (resolved at runtime)
- German translations for installation mode messages
- Comprehensive tests for installation mode integration

**Part 2 of 3 for XDG support**
- Depends on: PR #775 (must be merged first)
- Will rebase on main after merge of previous PR

### ⚙️ Type of Change
- [x] ✨ New feature (adds new functionality without breaking existing
usage)

## ✅ Checklist
- [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

* **New Features**
* Support for portable and XDG (system-wide) installation modes with
automatic detection and interactive first-run setup.
* Config and paths standardized so app stores config, downloads, logs,
and browser profiles in appropriate locations per mode.
  * Update checker improved for more reliable version/commit detection.

* **Chores**
* Moved dependency to runtime: platformdirs added to main dependencies.

* **Tests**
* Added comprehensive tests for installation modes, path utilities, and
related behaviors.

<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

@@ -112,6 +112,11 @@ kleinanzeigen_bot/__init__.py:
     " -> FAILED: Timeout while extending ad '%s': %s": " -> FEHLER: Zeitüberschreitung beim Verlängern der Anzeige '%s': %s"
     " -> FAILED: Could not persist extension for ad '%s': %s": " -> FEHLER: Verlängerung der Anzeige '%s' konnte nicht gespeichert werden: %s"
 
+  finalize_installation_mode:
+    "Config file: %s": "Konfigurationsdatei: %s"
+    "First run detected, prompting user for installation mode": "Erster Start erkannt, frage Benutzer nach Installationsmodus"
+    "Installation mode: %s": "Installationsmodus: %s"
+
   publish_ads:
     "Processing %s/%s: '%s' from [%s]...": "Verarbeite %s/%s: '%s' von [%s]..."
     "Skipping because ad is reserved": "Überspringen, da Anzeige reserviert ist"
@@ -240,7 +245,7 @@ kleinanzeigen_bot/__init__.py:
 kleinanzeigen_bot/extract.py:
 #################################################
   download_ad:
-    "Ensured ads directory exists at ./%s.": "Verzeichnis [%s] für Anzeige vorhanden."
+    "Using download directory: %s": "Verwende Download-Verzeichnis: %s"
 
   _download_and_save_image_sync:
     "Failed to download image %s: %s": "Fehler beim Herunterladen des Bildes %s: %s"