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/I18nKeys.h

@@ -422,6 +422,9 @@ enum class StrId : uint16_t {
   STR_DELETE_SERVER,
   STR_DELETE_CONFIRM,
   STR_OPDS_SERVERS,
+  STR_SAVE_HERE,
+  STR_SELECT_FOLDER,
+  STR_DOWNLOAD_PATH,
   // Sentinel - must be last
   _COUNT
 };