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

chat-summaries/2026-03-02_06-48-summary.md

@@ -0,0 +1,18 @@
+# OPDS Per-Server Default Save Directory
+
+## Task
+Add a configurable default download path per OPDS server. The directory picker now starts at the server's saved path instead of always at root. New servers default to "/".
+
+## Changes Made
+
+### Modified files
+- **`src/OpdsServerStore.h`** -- Added `downloadPath` field (default `"/"`) to `OpdsServer` struct.
+- **`src/OpdsServerStore.cpp`** -- Serialize/deserialize `download_path` in JSON. Existing servers without the field default to `"/"`.
+- **`src/activities/settings/OpdsSettingsActivity.cpp`** -- Added "Download Path" as field index 4 (shifted Delete to 5). Uses `DirectoryPickerActivity` as subactivity for folder selection. Displays current path in row value.
+- **`src/activities/util/DirectoryPickerActivity.h`** / **`.cpp`** -- Added `initialPath` constructor parameter (default `"/"`). `onEnter()` validates the path exists on disk and falls back to `"/"` if not.
+- **`src/activities/browser/OpdsBookBrowserActivity.cpp`** -- Passes `server.downloadPath` as `initialPath` when launching the directory picker.
+- **`lib/I18n/translations/*.yaml`** (all 9 languages) -- Added `STR_DOWNLOAD_PATH` with translations.
+- **`lib/I18n/I18nKeys.h`**, **`I18nStrings.h`**, **`I18nStrings.cpp`** -- Regenerated.
+
+## Follow-up Items
+- Test on device: verify settings UI shows/saves path, picker opens at saved path, fallback works when directory is removed