feat(ui): Add theme mode, style, and color settings

Add user-facing theme preferences with three independent axes:
- Theme mode (System/Dark/Light)
- Theme style (Default/Material You/Medium Contrast/High Contrast)
- Theme color (Blue/Green/Amber)

Includes safe Moshi enum fallback for corrupted preference values,
color palettes for all combinations, and window background sync
to prevent flash during navigation transitions.

app/src/test/java/eu/darken/capod/common/preferences/FlowPreferenceMoshiTest.kt

@@ -1,9 +1,12 @@
 package eu.darken.capod.common.preferences
 
 import com.squareup.moshi.JsonClass
+import com.squareup.moshi.JsonDataException
 import com.squareup.moshi.Moshi
+import eu.darken.capod.common.theming.ThemeMode
 import eu.darken.capod.main.core.MonitorMode
 import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.assertThrows
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.test.runTest
 import org.junit.jupiter.api.Test
@@ -121,4 +124,66 @@ class FlowPreferenceMoshiTest : BaseTest() {
         monitorMode.update { MonitorMode.MANUAL }
         monitorMode.value shouldBe MonitorMode.MANUAL
     }
+
+    @Test
+    fun `bad enum value throws without fallback`() = runTest {
+        val moshi = Moshi.Builder().build()
+        mockPreferences.edit().putString("theme.mode", "\"theme.mode.bogus\"").apply()
+
+        assertThrows<JsonDataException> {
+            mockPreferences.createFlowPreference(
+                key = "theme.mode",
+                defaultValue = ThemeMode.SYSTEM,
+                moshi = moshi,
+                onErrorFallbackToDefault = false,
+            )
+        }
+    }
+
+    @Test
+    fun `bad enum value returns default with fallback`() = runTest {
+        val moshi = Moshi.Builder().build()
+        mockPreferences.edit().putString("theme.mode", "\"theme.mode.bogus\"").apply()
+
+        val pref = mockPreferences.createFlowPreference(
+            key = "theme.mode",
+            defaultValue = ThemeMode.SYSTEM,
+            moshi = moshi,
+            onErrorFallbackToDefault = true,
+        )
+
+        pref.value shouldBe ThemeMode.SYSTEM
+        pref.flow.first() shouldBe ThemeMode.SYSTEM
+    }
+
+    @Test
+    fun `corrupt json returns default with fallback`() = runTest {
+        val moshi = Moshi.Builder().build()
+        mockPreferences.edit().putString("theme.mode", "not-json-at-all").apply()
+
+        val pref = mockPreferences.createFlowPreference(
+            key = "theme.mode",
+            defaultValue = ThemeMode.DARK,
+            moshi = moshi,
+            onErrorFallbackToDefault = true,
+        )
+
+        pref.value shouldBe ThemeMode.DARK
+    }
+
+    @Test
+    fun `valid enum roundtrips with fallback enabled`() = runTest {
+        val moshi = Moshi.Builder().build()
+        val pref = mockPreferences.createFlowPreference(
+            key = "theme.mode",
+            defaultValue = ThemeMode.SYSTEM,
+            moshi = moshi,
+            onErrorFallbackToDefault = true,
+        )
+
+        pref.value shouldBe ThemeMode.SYSTEM
+        pref.update { ThemeMode.DARK }
+        pref.value shouldBe ThemeMode.DARK
+        pref.flow.first() shouldBe ThemeMode.DARK
+    }
 }