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>

lib/GfxRenderer/BitmapHelpers.h

@@ -7,6 +7,7 @@ uint8_t quantize(int gray, int x, int y);
 uint8_t quantizeSimple(int gray);
 uint8_t quantize1bit(int gray, int x, int y);
 int adjustPixel(int gray);
+uint8_t quantizeNoiseDither(int gray, int x, int y);
 
 // 1-bit Atkinson dithering - better quality than noise dithering for thumbnails
 // Error distribution pattern (same as 2-bit but quantizes to 2 levels):