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/CrossPointSettings.h

@@ -31,6 +31,14 @@ class CrossPointSettings {
     INVERTED_BLACK_AND_WHITE = 2,
     SLEEP_SCREEN_COVER_FILTER_COUNT
   };
+  enum SLEEP_SCREEN_LETTERBOX_FILL {
+    LETTERBOX_NONE = 0,
+    LETTERBOX_SOLID = 1,
+    LETTERBOX_BLENDED = 2,
+    LETTERBOX_GRADIENT = 3,
+    SLEEP_SCREEN_LETTERBOX_FILL_COUNT
+  };
+  enum SLEEP_SCREEN_GRADIENT_DIR { GRADIENT_TO_WHITE = 0, GRADIENT_TO_BLACK = 1, SLEEP_SCREEN_GRADIENT_DIR_COUNT };
 
   // Status bar display type enum
   enum STATUS_BAR_MODE {
@@ -125,6 +133,10 @@ class CrossPointSettings {
   uint8_t sleepScreenCoverMode = FIT;
   // Sleep screen cover filter
   uint8_t sleepScreenCoverFilter = NO_FILTER;
+  // Sleep screen letterbox fill mode (None / Solid / Blended / Gradient)
+  uint8_t sleepScreenLetterboxFill = LETTERBOX_GRADIENT;
+  // Sleep screen gradient direction (towards white or black)
+  uint8_t sleepScreenGradientDir = GRADIENT_TO_WHITE;
   // Status bar settings
   uint8_t statusBar = FULL;
   // Text rendering settings