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/activities/settings/OpdsSettingsActivity.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <functional>
+
+#include "OpdsServerStore.h"
+#include "activities/ActivityWithSubactivity.h"
+#include "util/ButtonNavigator.h"
+
+/**
+ * Edit screen for a single OPDS server.
+ * Shows Name, URL, Username, Password fields and a Delete option.
+ * Used for both adding new servers and editing existing ones.
+ */
+class OpdsSettingsActivity final : public ActivityWithSubactivity {
+ public:
+  /**
+   * @param serverIndex Index into OpdsServerStore, or -1 for a new server
+   */
+  explicit OpdsSettingsActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                const std::function<void()>& onBack, int serverIndex = -1)
+      : ActivityWithSubactivity("OpdsSettings", renderer, mappedInput), onBack(onBack), serverIndex(serverIndex) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  void render(Activity::RenderLock&&) override;
+
+ private:
+  ButtonNavigator buttonNavigator;
+
+  size_t selectedIndex = 0;
+  const std::function<void()> onBack;
+  int serverIndex;
+  OpdsServer editServer;
+  bool isNewServer = false;
+
+  int getMenuItemCount() const;
+  void handleSelection();
+  void saveServer();
+};