feat: add directory picker for OPDS downloads with per-server default path

When downloading a book via OPDS, a directory picker now lets the user
choose the save location instead of always saving to the SD root. Each
OPDS server has a configurable default download path (persisted in
opds.json) that the picker opens to. Falls back to "/" if the saved
path no longer exists on disk.

- Add DirectoryPickerActivity (browse-only directory view with "Save Here")
- Add PICKING_DIRECTORY state to OpdsBookBrowserActivity
- Add downloadPath field to OpdsServer with JSON serialization
- Add Download Path setting to OPDS server edit screen
- Extract sortFileList() to StringUtils for shared use
- Add i18n strings: STR_SAVE_HERE, STR_SELECT_FOLDER, STR_DOWNLOAD_PATH

Made-with: Cursor

lib/I18n/translations/german.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Keine OPDS-Server konfiguriert"
 STR_DELETE_SERVER: "Server löschen"
 STR_DELETE_CONFIRM: "Diesen Server löschen?"
 STR_OPDS_SERVERS: "OPDS-Server"
+STR_SAVE_HERE: "Hier speichern"
+STR_SELECT_FOLDER: "Ordner auswählen"
+STR_DOWNLOAD_PATH: "Download-Pfad"