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
 };

lib/I18n/translations/czech.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Žádné OPDS servery nejsou nakonfigurovány"
 STR_DELETE_SERVER: "Smazat server"
 STR_DELETE_CONFIRM: "Smazat tento server?"
 STR_OPDS_SERVERS: "OPDS servery"
+STR_SAVE_HERE: "Uložit zde"
+STR_SELECT_FOLDER: "Vybrat složku"
+STR_DOWNLOAD_PATH: "Cesta ke stažení"

lib/I18n/translations/english.yaml

@@ -386,3 +386,6 @@ STR_NO_SERVERS: "No OPDS servers configured"
 STR_DELETE_SERVER: "Delete Server"
 STR_DELETE_CONFIRM: "Delete this server?"
 STR_OPDS_SERVERS: "OPDS Servers"
+STR_SAVE_HERE: "Save Here"
+STR_SELECT_FOLDER: "Select Folder"
+STR_DOWNLOAD_PATH: "Download Path"

lib/I18n/translations/french.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Aucun serveur OPDS configuré"
 STR_DELETE_SERVER: "Supprimer le serveur"
 STR_DELETE_CONFIRM: "Supprimer ce serveur ?"
 STR_OPDS_SERVERS: "Serveurs OPDS"
+STR_SAVE_HERE: "Enregistrer ici"
+STR_SELECT_FOLDER: "Sélectionner un dossier"
+STR_DOWNLOAD_PATH: "Chemin de téléchargement"

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"

lib/I18n/translations/portuguese.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Nenhum servidor OPDS configurado"
 STR_DELETE_SERVER: "Excluir servidor"
 STR_DELETE_CONFIRM: "Excluir este servidor?"
 STR_OPDS_SERVERS: "Servidores OPDS"
+STR_SAVE_HERE: "Salvar aqui"
+STR_SELECT_FOLDER: "Selecionar pasta"
+STR_DOWNLOAD_PATH: "Caminho de download"

lib/I18n/translations/romanian.yaml

@@ -325,3 +325,6 @@ STR_NO_SERVERS: "Niciun server OPDS configurat"
 STR_DELETE_SERVER: "Șterge serverul"
 STR_DELETE_CONFIRM: "Ștergi acest server?"
 STR_OPDS_SERVERS: "Servere OPDS"
+STR_SAVE_HERE: "Salvează aici"
+STR_SELECT_FOLDER: "Selectează dosar"
+STR_DOWNLOAD_PATH: "Cale descărcare"

lib/I18n/translations/russian.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Нет настроенных серверов OPDS"
 STR_DELETE_SERVER: "Удалить сервер"
 STR_DELETE_CONFIRM: "Удалить этот сервер?"
 STR_OPDS_SERVERS: "Серверы OPDS"
+STR_SAVE_HERE: "Сохранить здесь"
+STR_SELECT_FOLDER: "Выбрать папку"
+STR_DOWNLOAD_PATH: "Путь загрузки"

lib/I18n/translations/spanish.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "No hay servidores OPDS configurados"
 STR_DELETE_SERVER: "Eliminar servidor"
 STR_DELETE_CONFIRM: "¿Eliminar este servidor?"
 STR_OPDS_SERVERS: "Servidores OPDS"
+STR_SAVE_HERE: "Guardar aquí"
+STR_SELECT_FOLDER: "Seleccionar carpeta"
+STR_DOWNLOAD_PATH: "Ruta de descarga"

lib/I18n/translations/swedish.yaml

@@ -350,3 +350,6 @@ STR_NO_SERVERS: "Inga OPDS-servrar konfigurerade"
 STR_DELETE_SERVER: "Ta bort server"
 STR_DELETE_CONFIRM: "Ta bort denna server?"
 STR_OPDS_SERVERS: "OPDS-servrar"
+STR_SAVE_HERE: "Spara här"
+STR_SELECT_FOLDER: "Välj mapp"
+STR_DOWNLOAD_PATH: "Nedladdningssökväg"