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/2026-02-12_merge-master-into-mod-master.md

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+# Merge master into mod/master
+
+**Date:** 2026-02-12
+
+## Task
+
+Compare upstream `master` (14 new commits) with `mod/master` (2 mod commits) since their common ancestor (`6202bfd` — release/1.0.0 merge), assess merge risk, and perform the merge.
+
+## Branch Summary
+
+### Upstream (`master`) — 14 commits, 47 files, ~6000 lines
+- Unified navigation handling with ButtonNavigator utility
+- Italian hyphenation support
+- Natural sort in file browser
+- Auto WiFi reconnect to last network
+- Extended Python debugging monitor
+- More power saving on idle
+- OPDS fixes (absolute URLs, prevent sleep during download)
+- Uniform debug message formatting (millis() timestamps)
+- File browser Back/Home label fix, GPIO trigger fix
+- USER_GUIDE.md updates
+
+### Mod (`mod/master`) — 2 commits, 10 files, ~588 lines
+- `.gitignore` tweaks for mod fork
+- Sleep screen letterbox fill and image upscaling feature
+
+## Conflict Resolution
+
+Single conflict in `src/activities/boot_sleep/SleepActivity.cpp`:
+- **Upstream** changed `Serial.println` → `Serial.printf("[%lu] [SLP] ...\n", millis())` for uniform debug logging
+- **Mod** had already adopted this format in new code, but the original lines it modified were the old format
+- **Resolution:** Kept mod's `renderBitmapSleepScreen(bitmap, edgeCachePath)` call with upstream's `millis()` log format
+
+## Result
+
+Merge commit: `182c236`
+
+## Follow-up
+
+- Test sleep screen behavior end-to-end (letterbox fill + upstream idle power saving changes)
+- Verify new upstream features (navigation, WiFi auto-connect) work alongside mod changes