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/GfxRenderer.h

@@ -77,6 +77,7 @@ class GfxRenderer {
 
   // Drawing
   void drawPixel(int x, int y, bool state = true) const;
+  void drawPixelGray(int x, int y, uint8_t val2bit) const;
   void drawLine(int x1, int y1, int x2, int y2, bool state = true) const;
   void drawLine(int x1, int y1, int x2, int y2, int lineWidth, bool state) const;
   void drawArc(int maxRadius, int cx, int cy, int xDir, int yDir, int lineWidth, bool state) const;