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

src/util/StringUtils.h

@@ -3,6 +3,7 @@
 #include <WString.h>
 
 #include <string>
+#include <vector>
 
 namespace StringUtils {
 
@@ -19,4 +20,10 @@ std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
 bool checkFileExtension(const std::string& fileName, const char* extension);
 bool checkFileExtension(const String& fileName, const char* extension);
 
+/**
+ * Sort a file/directory list with directories first, using case-insensitive natural sort.
+ * Directory entries are identified by a trailing '/'.
+ */
+void sortFileList(std::vector<std::string>& entries);
+
 }  // namespace StringUtils