feat: port upstream OPDS improvements (PRs #1207, #1209)

Port two upstream PRs:

- PR #1207: Replace manual chunked download loop with
  HTTPClient::writeToStream via a FileWriteStream adapter, improving
  reliability for OPDS file downloads including chunked transfers.

- PR #1209: Add support for multiple OPDS servers with a new
  OpdsServerStore (JSON persistence with MAC-based password obfuscation),
  OpdsServerListActivity and OpdsSettingsActivity UIs, per-server
  credentials passed to HttpDownloader, web UI management endpoints,
  and migration from legacy single-server settings.

Made-with: Cursor

src/SettingsList.h

@@ -191,13 +191,5 @@ inline std::vector<SettingInfo> getSettingsList() {
             KOREADER_STORE.saveToFile();
           },
           "koMatchMethod", StrId::STR_KOREADER_SYNC),
-
-      // --- OPDS Browser (web-only, uses CrossPointSettings char arrays) ---
-      SettingInfo::String(StrId::STR_OPDS_SERVER_URL, SETTINGS.opdsServerUrl, sizeof(SETTINGS.opdsServerUrl),
-                          "opdsServerUrl", StrId::STR_OPDS_BROWSER),
-      SettingInfo::String(StrId::STR_USERNAME, SETTINGS.opdsUsername, sizeof(SETTINGS.opdsUsername), "opdsUsername",
-                          StrId::STR_OPDS_BROWSER),
-      SettingInfo::String(StrId::STR_PASSWORD, SETTINGS.opdsPassword, sizeof(SETTINGS.opdsPassword), "opdsPassword",
-                          StrId::STR_OPDS_BROWSER),
   };
 }