User theme: customizable bubble colors + background (color/image) in Settings → Appearance

- UserTheme model (ARGB ints) + LocalUserTheme, persisted via SecureStore
- HSV color picker dialog (Apply keeps open, OK applies+closes, Cancel)
- Background: solid color or image (SAF on Android, JFileChooser on desktop)
- Reasoning block: 0.6 black overlay on agent bubble color (adapts automatically)
- Contrast-aware text on bubbles + pinned contentColor on root Surface

app/shared/src/androidMain/kotlin/iris/platform/AndroidMedia.kt

@@ -82,6 +82,31 @@ actual fun MediaFilePicker(onPicked: (PickedFile?) -> Unit) {
     }
 }
 
+@Composable
+actual fun ImageFilePicker(onPicked: (PickedFile?) -> Unit) {
+    val context = LocalContext.current
+    val launcher = rememberLauncherForActivityResult(ActivityResultContracts.GetContent()) { uri ->
+        if (uri == null) {
+            onPicked(null)
+            return@rememberLauncherForActivityResult
+        }
+        try {
+            val mime = context.contentResolver.getType(uri) ?: "image/jpeg"
+            val name = queryDisplayName(context, uri) ?: "background"
+            // Persistent (filesDir), unlike the upload staging dir in cacheDir.
+            val dir = File(context.filesDir, "theme").apply { mkdirs() }
+            val target = File(dir, "background${extForMime(mime)}")
+            context.contentResolver.openInputStream(uri)!!.use { input ->
+                target.outputStream().use { output -> input.copyTo(output) }
+            }
+            onPicked(PickedFile(target.absolutePath, name, mime, target.length()))
+        } catch (e: Exception) {
+            onPicked(null)
+        }
+    }
+    Button(onClick = { launcher.launch("image/*") }) { Text("Choose image…") }
+}
+
 private fun queryDisplayName(context: Context, uri: Uri): String? {
     context.contentResolver.query(uri, null, null, null, null)?.use { c ->
         val idx = c.getColumnIndex(OpenableColumns.DISPLAY_NAME)
@@ -102,6 +127,22 @@ actual suspend fun loadMediaImage(path: String): ImageBitmap? = withContext(Disp
     }
 }
 
+actual suspend fun loadScaledImage(path: String, maxSize: Int): ImageBitmap? = withContext(Dispatchers.IO) {
+    try {
+        // inJustDecodeBounds fills [bounds] and returns null by design — only
+        // the outWidth/outHeight are meaningful here.
+        val bounds = BitmapFactory.Options().apply { inJustDecodeBounds = true }
+        BitmapFactory.decodeFile(path, bounds)
+        if (bounds.outWidth <= 0 || bounds.outHeight <= 0) return@withContext null
+        var sample = 1
+        while (maxOf(bounds.outWidth, bounds.outHeight) / sample > maxSize) sample *= 2
+        val opts = BitmapFactory.Options().apply { inSampleSize = sample }
+        BitmapFactory.decodeFile(path, opts)?.asImageBitmap()
+    } catch (e: Exception) {
+        null
+    }
+}
+
 @Composable
 actual fun MediaPlayerView(media: MediaItem, modifier: Modifier) {
     val path = media.localPath

app/shared/src/androidMain/kotlin/iris/platform/AndroidSecureStore.kt

@@ -6,6 +6,7 @@ import android.os.Build
 import androidx.security.crypto.EncryptedSharedPreferences
 import androidx.security.crypto.MasterKey
 import iris.data.SecureStore
+import iris.ui.theme.UserTheme
 import java.util.UUID
 
 /**
@@ -113,6 +114,26 @@ class AndroidSecureStore(context: Context) : SecureStore {
         get() = prefs.getBoolean(KEY_REASONING_AUTO_COLLAPSE, true)
         set(value) = prefs.edit().putBoolean(KEY_REASONING_AUTO_COLLAPSE, value).apply()
 
+    override var userBubbleColor: Int
+        get() = prefs.getInt(KEY_USER_BUBBLE_COLOR, UserTheme.DEFAULT_USER_BUBBLE)
+        set(value) = prefs.edit().putInt(KEY_USER_BUBBLE_COLOR, value).apply()
+
+    override var agentBubbleColor: Int
+        get() = prefs.getInt(KEY_AGENT_BUBBLE_COLOR, UserTheme.DEFAULT_AGENT_BUBBLE)
+        set(value) = prefs.edit().putInt(KEY_AGENT_BUBBLE_COLOR, value).apply()
+
+    override var backgroundMode: String
+        get() = prefs.getString(KEY_BACKGROUND_MODE, "color").orEmpty().ifBlank { "color" }
+        set(value) = prefs.edit().putString(KEY_BACKGROUND_MODE, value).apply()
+
+    override var backgroundColor: Int
+        get() = prefs.getInt(KEY_BACKGROUND_COLOR, UserTheme.DEFAULT_BACKGROUND)
+        set(value) = prefs.edit().putInt(KEY_BACKGROUND_COLOR, value).apply()
+
+    override var backgroundImagePath: String
+        get() = prefs.getString(KEY_BACKGROUND_IMAGE_PATH, "").orEmpty()
+        set(value) = prefs.edit().putString(KEY_BACKGROUND_IMAGE_PATH, value).apply()
+
     override fun savePairing(url: String, token: String) {
         serverUrl = url
         this.token = token
@@ -136,5 +157,10 @@ class AndroidSecureStore(context: Context) : SecureStore {
         const val KEY_TOOL_DETAIL = "tool_detail"
         const val KEY_STREAMING_ENABLED = "streaming_enabled"
         const val KEY_REASONING_AUTO_COLLAPSE = "reasoning_auto_collapse"
+        const val KEY_USER_BUBBLE_COLOR = "user_bubble_color"
+        const val KEY_AGENT_BUBBLE_COLOR = "agent_bubble_color"
+        const val KEY_BACKGROUND_MODE = "background_mode"
+        const val KEY_BACKGROUND_COLOR = "background_color"
+        const val KEY_BACKGROUND_IMAGE_PATH = "background_image_path"
     }
 }