fix #4150: display 0°C instead of -0°C for near-zero negative temperatures (#4186)

Signed-off-by: lowi <75674438+lohwasser@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

core/model/src/main/kotlin/org/meshtastic/core/model/util/UnitConversions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025 Meshtastic LLC
+ * Copyright (c) 2025-2026 Meshtastic LLC
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -14,21 +14,31 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-
 package org.meshtastic.core.model.util
 
 import kotlin.math.ln
+import kotlin.math.roundToInt
 
 object UnitConversions {
 
     @Suppress("MagicNumber")
     fun celsiusToFahrenheit(celsius: Float): Float = (celsius * 1.8F) + 32
 
-    fun Float.toTempString(isFahrenheit: Boolean) = if (isFahrenheit) {
-        val fahrenheit = celsiusToFahrenheit(this)
-        "%.0f°F".format(fahrenheit)
-    } else {
-        "%.0f°C".format(this)
+    /** Formats temperature as a string with the unit suffix. */
+    fun Float.toTempString(isFahrenheit: Boolean): String {
+        val temp = if (isFahrenheit) celsiusToFahrenheit(this) else this
+        val unit = if (isFahrenheit) "F" else "C"
+
+        // Convoluted calculation due to edge case: rounding negative values.
+        // We round the absolute value using roundToInt() (banker's rounding), then reapply the sign so values
+        val absoluteTemp: Float = kotlin.math.abs(temp)
+        val roundedAbsoluteTemp: Int = absoluteTemp.roundToInt()
+
+        val isZero = roundedAbsoluteTemp == 0
+        val isPositive = kotlin.math.sign(temp) > 0
+        val sign: String = if (isPositive || isZero) "" else "-"
+
+        return "$sign$roundedAbsoluteTemp°$unit"
     }
 
     /**