feat: update check (#561)

feat(update-check): add robust update check with interval support, state management, and CLI integration

- Implement version and interval-based update checks with configurable settings
- Add CLI command `kleinanzeigen-bot update-check` for manual checks
- Introduce state file with versioning, UTC timestamps, and migration logic
- Validate and normalize intervals (1d–4w) with fallback for invalid values
- Ensure correct handling of timezones and elapsed checks
- Improve error handling, logging, and internationalization (i18n)
- Add comprehensive test coverage for config, interval logic, migration, and CLI
- Align default config, translations, and schema with new functionality
- Improve help command UX by avoiding config/log loading for `--help`
- Update documentation and README with full feature overview

README.md

@@ -296,6 +296,17 @@ browser:
   user_data_dir: "" # see https://github.com/chromium/chromium/blob/main/docs/user_data_dir.md
   profile_name: ""
 
+# update check configuration
+update_check:
+  enabled: true  # Enable/disable update checks
+  channel: latest  # One of: latest, prerelease
+  interval: 7d    # Check interval (e.g. 7d for 7 days)
+                  # If the interval is invalid, too short (<1d), or too long (>30d),
+                  # the bot logs a warning and uses a default interval for this run:
+                  #   - 1d for 'prerelease' channel
+                  #   - 7d for 'latest' channel
+                  # The config file is not changed automatically; please fix your config to avoid repeated warnings.
+
 # login credentials
 login:
   username: ""