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/OpdsServerStore.cpp

@@ -80,6 +80,7 @@ bool OpdsServerStore::saveToFile() const {
     obj["url"] = server.url;
     obj["username"] = server.username;
     obj["password_obf"] = obfuscateToBase64(server.password);
+    obj["download_path"] = server.downloadPath;
   }
 
   String json;
@@ -114,6 +115,7 @@ bool OpdsServerStore::loadFromFile() {
           server.password = obj["password"] | std::string("");
           if (!server.password.empty()) needsResave = true;
         }
+        server.downloadPath = obj["download_path"] | std::string("/");
         servers.push_back(std::move(server));
       }