feat: Sleep screen letterbox fill and image upscaling

Add configurable letterbox fill for sleep screen cover images that don't
match the display aspect ratio. Four fill modes are available: Solid
(single dominant edge shade), Blended (per-pixel edge colors), Gradient
(edge colors interpolated toward white/black), and None.

Enable upscaling of cover images smaller than the display in Fit mode by
modifying drawBitmap/drawBitmap1Bit to support both up and downscaling
via a unified block-fill approach.

Edge sampling data is cached to .crosspoint alongside the cover BMP to
avoid redundant bitmap scanning on subsequent sleeps. Cache is validated
against screen dimensions and auto-regenerated when stale.

New settings: Letterbox Fill (None/Solid/Blended/Gradient) and Gradient
Direction (To White/To Black).

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

src/SettingsList.h

@@ -18,6 +18,10 @@ inline std::vector<SettingInfo> getSettingsList() {
                         "sleepScreenCoverMode", "Display"),
       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"),
       SettingInfo::Enum(
           "Status Bar", &CrossPointSettings::statusBar,
           {"None", "No Progress", "Full w/ Percentage", "Full w/ Book Bar", "Book Bar Only", "Full w/ Chapter Bar"},