feat(battery): Add a shared battery tier for level colours

The overview duplicates the same 30%/15% thresholds in four places, with two
subtly different unknown checks: the capsule and the pod gauge accept any
percent >= 0f, so Float.POSITIVE_INFINITY renders as a full, healthy ring.
batteryTier() settles that on the finite check the other two already use.

app/src/main/java/eu/darken/capod/monitor/core/battery/BatteryTier.kt

@@ -0,0 +1,23 @@
+package eu.darken.capod.monitor.core.battery
+
+/** Battery bands behind the overview's colour semantics. */
+enum class BatteryTier {
+    UNKNOWN,
+    CRITICAL,
+    WARN,
+    GOOD,
+}
+
+/**
+ * Maps a battery fraction onto a [BatteryTier]. Only a finite, non-negative fraction has a level;
+ * anything else is [BatteryTier.UNKNOWN]. Values above 1 count as full.
+ */
+fun batteryTier(percent: Float): BatteryTier {
+    if (!percent.isFinite() || percent < 0f) return BatteryTier.UNKNOWN
+    val clamped = percent.coerceIn(0f, 1f)
+    return when {
+        clamped > 0.30f -> BatteryTier.GOOD
+        clamped >= 0.15f -> BatteryTier.WARN
+        else -> BatteryTier.CRITICAL
+    }
+}

app/src/test/java/eu/darken/capod/monitor/core/battery/BatteryTierTest.kt

@@ -0,0 +1,50 @@
+package eu.darken.capod.monitor.core.battery
+
+import eu.darken.capod.pods.core.apple.ble.BATTERY_UNKNOWN
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+import testhelpers.BaseTest
+
+class BatteryTierTest : BaseTest() {
+
+    @Test
+    fun `unknown battery has no tier`() {
+        batteryTier(BATTERY_UNKNOWN) shouldBe BatteryTier.UNKNOWN
+        batteryTier(-0.01f) shouldBe BatteryTier.UNKNOWN
+        batteryTier(-100f) shouldBe BatteryTier.UNKNOWN
+    }
+
+    @Test
+    fun `non-finite values have no tier`() {
+        batteryTier(Float.NaN) shouldBe BatteryTier.UNKNOWN
+        batteryTier(Float.POSITIVE_INFINITY) shouldBe BatteryTier.UNKNOWN
+        batteryTier(Float.NEGATIVE_INFINITY) shouldBe BatteryTier.UNKNOWN
+    }
+
+    @Test
+    fun `values at and above full are good`() {
+        batteryTier(1f) shouldBe BatteryTier.GOOD
+        batteryTier(1.5f) shouldBe BatteryTier.GOOD
+        batteryTier(Float.MAX_VALUE) shouldBe BatteryTier.GOOD
+    }
+
+    @Test
+    fun `the good boundary is exclusive`() {
+        batteryTier(0.30f) shouldBe BatteryTier.WARN
+        batteryTier(0.30f + 0.0001f) shouldBe BatteryTier.GOOD
+        batteryTier(0.30f - 0.0001f) shouldBe BatteryTier.WARN
+    }
+
+    @Test
+    fun `the warn boundary is inclusive`() {
+        batteryTier(0.15f) shouldBe BatteryTier.WARN
+        batteryTier(0.15f + 0.0001f) shouldBe BatteryTier.WARN
+        batteryTier(0.15f - 0.0001f) shouldBe BatteryTier.CRITICAL
+    }
+
+    @Test
+    fun `an empty battery is critical`() {
+        batteryTier(0f) shouldBe BatteryTier.CRITICAL
+        batteryTier(0.05f) shouldBe BatteryTier.CRITICAL
+    }
+}