mod: Phase 1 - bring forward mod-exclusive files with ActivityManager migration

Brings ~55 mod-exclusive files to the upstream-based mod/master-resync branch:

Activities (migrated to new ActivityManager pattern):
- Clock/Time: SetTimeActivity, SetTimezoneOffsetActivity, NtpSyncActivity
- Dictionary: DictionaryDefinitionActivity, DictionarySuggestionsActivity,
  DictionaryWordSelectActivity, LookedUpWordsActivity
- Bookmark: EpubReaderBookmarkSelectionActivity
- Book management: BookManageMenuActivity, EndOfBookMenuActivity
- OPDS: OpdsServerListActivity, OpdsSettingsActivity
- Utility: DirectoryPickerActivity, NumericStepperActivity

Utilities (unchanged):
- BookManager, BookSettings, BookmarkStore, BootNtpSync
- Dictionary, LookupHistory, TimeSync, OpdsServerStore

Libraries: PlaceholderCover, TableData, ChapterXPathIndexer
Scripts: inject_mod_version, generate_book_icon, preview_placeholder_cover
Docs: KOReader sync XPath mapping

Migration changes:
- ActivityWithSubactivity -> Activity base class
- Callback constructors -> finish()/setResult() pattern
- enterNewActivity() -> startActivityForResult()
- Activity::RenderLock&& -> RenderLock&&

These files won't compile yet - they reference mod settings and I18n
strings that will be added in subsequent phases.

Made-with: Cursor

src/activities/settings/OpdsSettingsActivity.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include "OpdsServerStore.h"
+#include "activities/Activity.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 Activity {
+ public:
+  /**
+   * @param serverIndex Index into OpdsServerStore, or -1 for a new server
+   */
+  explicit OpdsSettingsActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, int serverIndex = -1)
+      : Activity("OpdsSettings", renderer, mappedInput), serverIndex(serverIndex) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  void render(RenderLock&&) override;
+
+ private:
+  ButtonNavigator buttonNavigator;
+
+  size_t selectedIndex = 0;
+  int serverIndex;
+  OpdsServer editServer;
+  bool isNewServer = false;
+
+  int getMenuItemCount() const;
+  void handleSelection();
+  void saveServer();
+};