feat: add core XDG path resolution module (#775)

## ℹ️ Description
Core module for XDG Base Directory specification support.

- Link to the related issue(s): N/A (new feature)
- Adds portable and XDG installation mode path resolution

## 📋 Changes Summary

- New `xdg_paths.py` module with 11 path resolution functions
- Comprehensive test suite (32 tests, 95% coverage)
- German translations for all user-facing strings
- Moved `platformdirs` from dev to runtime dependencies

**Part 1 of 3 for XDG support**
- Depends on: None
- Preserves: extend command, ContactDefaults.location

### ⚙️ 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**
* Added support for portable and XDG-standard installation modes for
flexible config, cache, and state storage.

* **Chores**
* Added a runtime dependency to handle platform-specific directory
locations.

* **Tests**
* Added comprehensive unit tests covering path resolution,
installation-mode detection, interactive prompts, and Unicode path
handling.

<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

@@ -616,3 +616,21 @@ kleinanzeigen_bot/model/update_check_state.py:
     "Interval too short: %s. Minimum interval is 1d. Using default interval for this run.": "Intervall zu kurz: %s. Das Mindestintervall beträgt 1 Tag. Es wird das Standardintervall für diesen Durchlauf verwendet."
     "Invalid interval format or unsupported unit: %s. Using default interval for this run.": "Ungültiges Intervallformat oder nicht unterstützte Einheit: %s. Es wird das Standardintervall für diesen Durchlauf verwendet."
     "Negative interval: %s. Minimum interval is 1d. Using default interval for this run.": "Negatives Intervall: %s. Das Mindestintervall beträgt 1 Tag. Es wird das Standardintervall für diesen Durchlauf verwendet."
+
+#################################################
+kleinanzeigen_bot/utils/xdg_paths.py:
+#################################################
+  _ensure_directory:
+    "Failed to create %s %s: %s": "Fehler beim Erstellen von %s %s: %s"
+  detect_installation_mode:
+    "Detected installation mode: %s": "Erkannter Installationsmodus: %s"
+    "No existing installation found": "Keine bestehende Installation gefunden"
+  prompt_installation_mode:
+    "Non-interactive mode detected, defaulting to portable installation": "Nicht-interaktiver Modus erkannt, Standard-Installation: portabel"
+    "Choose installation type:": "Installationstyp wählen:"
+    "[1] Portable (current directory)": "[1] Portabel (aktuelles Verzeichnis)"
+    "[2] System-wide (XDG directories)": "[2] Systemweit (XDG-Verzeichnisse)"
+    "Enter 1 or 2: ": "1 oder 2 eingeben: "
+    "Defaulting to portable installation mode": "Standard-Installationsmodus: portabel"
+    "User selected installation mode: %s": "Benutzer hat Installationsmodus gewählt: %s"
+    "Invalid choice. Please enter 1 or 2.": "Ungültige Auswahl. Bitte 1 oder 2 eingeben."