refactor: Revert letterbox fill to Dithered/Solid/None with edge caching

Simplify letterbox fill modes back to Dithered (default), Solid, and
None. Remove the Extend Edges mode and all per-pixel edge replication
code. Restore Bayer ordered dithering for the Dithered fill mode.

Re-introduce edge average caching so cover edge computations persist
across sleep cycles, stored as a small binary file alongside the cover
BMP.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/boot_sleep/SleepActivity.h

@@ -1,4 +1,5 @@
 #pragma once
+
 #include <string>
 
 #include "../Activity.h"