Implement getTextWidth for display classes

- Added getTextWidth method to DisplayDriver interface
- Implemented getTextWidth in all display classes
- Updated examples to use getTextWidth directly

src/helpers/ui/GxEPDDisplay.h

@@ -47,5 +47,6 @@ public:
   void fillRect(int x, int y, int w, int h) override;
   void drawRect(int x, int y, int w, int h) override;
   void drawXbm(int x, int y, const uint8_t* bits, int w, int h) override;
+  uint16_t getTextWidth(const char* str) override;
   void endFrame() override;
 };