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/activities/boot_sleep/SleepActivity.h

@@ -1,4 +1,6 @@
 #pragma once
+#include <string>
+
 #include "../Activity.h"
 
 class Bitmap;
@@ -13,6 +15,6 @@ class SleepActivity final : public Activity {
   void renderDefaultSleepScreen() const;
   void renderCustomSleepScreen() const;
   void renderCoverSleepScreen() const;
-  void renderBitmapSleepScreen(const Bitmap& bitmap) const;
+  void renderBitmapSleepScreen(const Bitmap& bitmap, const std::string& edgeCachePath = "") const;
   void renderBlankSleepScreen() const;
 };