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/2026-02-13_letterbox-fill-redesign.md

@@ -0,0 +1,42 @@
+# Letterbox Fill Redesign
+
+**Date:** 2026-02-13
+**Task:** Strip out the 5-mode letterbox edge fill system and replace with a simplified 3-mode design
+
+## Changes Made
+
+### Problem
+The existing letterbox fill feature had 5 modes (None, Solid, Blended, Gradient, Matched) with ~300 lines of complex code including per-pixel edge arrays, malloc'd buffers, binary edge caching, framebuffer-level column/row copying, and a gradient direction sub-setting. Several modes introduced visual corruption that couldn't be resolved.
+
+### New Design
+Simplified to 3 modes:
+- **None** -- no fill
+- **Solid** -- computes dominant edge color, snaps to nearest of 4 e-ink levels (black/dark gray/light gray/white), fills uniformly
+- **Blended** -- computes dominant edge color, fills with exact gray value using noise dithering for smooth approximation
+
+### Files Changed
+
+1. **`src/CrossPointSettings.h`** -- Removed `LETTERBOX_GRADIENT`, `LETTERBOX_MATCHED` enum values; removed `SLEEP_SCREEN_GRADIENT_DIR` enum and `sleepScreenGradientDir` member; changed default to `LETTERBOX_NONE`
+
+2. **`src/SettingsList.h`** -- Trimmed Letterbox Fill options to `{None, Solid, Blended}`; removed Gradient Direction setting entry
+
+3. **`src/CrossPointSettings.cpp`** -- Removed `sleepScreenGradientDir` from write path; added dummy read for backward compatibility with old settings files; decremented `SETTINGS_COUNT` from 32 to 31
+
+4. **`src/activities/boot_sleep/SleepActivity.cpp`** -- Major rewrite:
+   - Removed: `LetterboxGradientData` struct, `loadEdgeCache()`/`saveEdgeCache()`, `sampleBitmapEdges()`, `copyEdgeRowsToLetterbox()`, old `drawLetterboxFill()`
+   - Added: `LetterboxFillData` struct (2 bytes vs arrays), `snapToEinkLevel()`, `computeEdgeAverages()` (running sums only, no malloc), simplified `drawLetterboxFill()`
+   - Cleaned `renderBitmapSleepScreen()`: removed matched/gradient logic, edge cache paths, unused `scaledWidth`/`scaledHeight`
+   - Cleaned `renderCoverSleepScreen()`: removed edge cache path derivation
+   - Removed unused includes (`<Serialization.h>`, `<cstring>`), added `<cmath>`
+
+5. **`src/activities/boot_sleep/SleepActivity.h`** -- Removed `edgeCachePath` parameter from `renderBitmapSleepScreen()` signature; removed unused `<string>` include
+
+### Backward Compatibility
+- Enum values 0/1/2 (None/Solid/Blended) unchanged -- existing settings preserved
+- Old Gradient (3) or Matched (4) values rejected by `readAndValidate`, falling back to default (None)
+- Old `sleepScreenGradientDir` byte consumed via dummy read during settings load
+- Orphaned `_edges.bin` cache files on SD cards are harmless
+
+## Follow-up Items
+- Test all 3 fill modes on device with various cover aspect ratios
+- Consider cleaning up orphaned `_edges.bin` files (optional, low priority)