feat: add runtime display rotation setting for e-ink builds

Adds Settings > Display > Rotation (0°/90°/180°/270°) that persists to
NodePrefs and is applied on startup. Falls back to compile-time
DISPLAY_ROTATION when the field is missing from an older prefs file.

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

src/helpers/ui/GxEPDDisplay.h

@@ -84,5 +84,6 @@ public:
   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 setDisplayRotation(uint8_t rot) override;
   void endFrame() override;
 };