feat: add BAR unit support to TPMS RX app (#2988)

- Add bar() conversion method to Pressure class
- Convert pressure units system from boolean to enum (kPa, PSI, BAR)
- Add BAR option to pressure unit selector in UI
- Update settings manager to persist pressure unit preference
- Use ternary operators for compact pressure formatting

firmware/common/units.hpp

@@ -48,6 +48,10 @@ class Pressure {
         return kpa_ * 1000 / 6895;
     }
 
+    int bar() const {
+        return kpa_ / 100;
+    }
+
    private:
     int16_t kpa_;
 };