add settings for reader screen margin (#223)

## Summary

* **What is the goal of this PR?** 
* This PR adds a setting to control the top left and right margins of
the reader screen in 4 sizes (5, 10, 20, 40 pt?) and defaults to `SMALL`
which is equivalent to the fixed margin of 5 that was already in use
before.
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks, specific areas to
focus on).

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/CrossPointSettings.h

@@ -75,6 +75,9 @@ class CrossPointSettings {
   // E-ink refresh frequency (default 15 pages)
   uint8_t refreshFrequency = REFRESH_15;
 
+  // Reader screen margin settings
+  uint8_t screenMargin = 5;
+
   ~CrossPointSettings() = default;
 
   // Get singleton instance
@@ -89,6 +92,7 @@ class CrossPointSettings {
   float getReaderLineCompression() const;
   unsigned long getSleepTimeoutMs() const;
   int getRefreshFrequency() const;
+  int getReaderScreenMargin() const;
 };
 
 // Helper macro to access settings