feat(eink): full refresh interval setting in Settings › Display

Add "Full rfsh" setting (e-ink only) with options off/5/10/20/30
partial refreshes between full refreshes. Prevents ghosting on panels
that accumulate artifacts after many partial updates.

GxEPDDisplay counts partial refreshes and calls display.display(false)
every N frames when the interval is set. Persisted in DataStore and
applied at startup via applyFullRefreshInterval().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/ui/DisplayDriver.h

@@ -228,5 +228,6 @@ public:
   
   virtual void setBrightness(uint8_t level) { }  // level 0-4 (min to max), no-op default
   virtual void setDisplayRotation(uint8_t rot) { }  // 0-3, no-op for fixed-orientation displays
+  virtual void setFullRefreshInterval(uint8_t n) { }  // e-ink: do full refresh every n partial refreshes (0=never)
   virtual void endFrame() = 0;
 };