Sensors page: show placeholders for all connected sensors; support all LPP types

- SensorManager::getAvailableLPPTypes() reports which LPP types will be
  present based on initialized sensor flags (temp, humidity, pressure, etc.)
- Sensors page renders one row per available type; shows "--" for any sensor
  that hasn't produced a reading yet instead of hiding the row entirely
- LPPReader: add readLuminosity, readPercentage, readConcentration, readDistance
- All known LPP types now have proper names and units in the sensor display

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

src/helpers/SensorManager.h

@@ -23,6 +23,7 @@ public:
   virtual const char* getSettingValue(int i) const { return NULL; }
   virtual bool setSettingValue(const char* name, const char* value) { return false; }
   virtual LocationProvider* getLocationProvider() { return NULL; }
+  virtual int getAvailableLPPTypes(uint8_t* types, int max_count) const { return 0; }
 
   // Helper functions to manage setting by keys (useful in many places ...)
   const char* getSettingByKey(const char* key) {