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/SettingsList.h

@@ -19,9 +19,7 @@ inline std::vector<SettingInfo> getSettingsList() {
       SettingInfo::Enum("Sleep Screen Cover Filter", &CrossPointSettings::sleepScreenCoverFilter,
                         {"None", "Contrast", "Inverted"}, "sleepScreenCoverFilter", "Display"),
       SettingInfo::Enum("Letterbox Fill", &CrossPointSettings::sleepScreenLetterboxFill,
-                        {"None", "Solid", "Blended", "Gradient"}, "sleepScreenLetterboxFill", "Display"),
-      SettingInfo::Enum("Gradient Direction", &CrossPointSettings::sleepScreenGradientDir, {"To White", "To Black"},
-                        "sleepScreenGradientDir", "Display"),
+                        {"Dithered", "Solid", "None"}, "sleepScreenLetterboxFill", "Display"),
       SettingInfo::Enum(
           "Status Bar", &CrossPointSettings::statusBar,
           {"None", "No Progress", "Full w/ Percentage", "Full w/ Book Bar", "Book Bar Only", "Full w/ Chapter Bar"},