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

lib/I18n/I18nKeys.h

@@ -416,6 +416,12 @@ enum class StrId : uint16_t {
   STR_ACTION_FAILED,
   STR_BACK_TO_BEGINNING,
   STR_CLOSE_MENU,
+  STR_ADD_SERVER,
+  STR_SERVER_NAME,
+  STR_NO_SERVERS,
+  STR_DELETE_SERVER,
+  STR_DELETE_CONFIRM,
+  STR_OPDS_SERVERS,
   // Sentinel - must be last
   _COUNT
 };