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/commonMain/kotlin/iris/IrisApp.kt

@@ -1,14 +1,18 @@
 package iris
 
-import androidx.compose.foundation.background
+import androidx.compose.foundation.Image
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material3.Surface
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.layout.ContentScale
 import iris.data.SecureStore
 import iris.net.GatewayClient
 import iris.platform.setActiveController
@@ -17,6 +21,8 @@ import iris.ui.screens.ChatScreen
 import iris.ui.screens.ConnectScreen
 import iris.ui.theme.IrisColors
 import iris.ui.theme.IrisTheme
+import iris.ui.theme.LocalUserTheme
+import iris.ui.theme.rememberBackgroundImage
 
 /**
  * Root composable shared by the Android and Desktop shells.
@@ -47,24 +53,50 @@ fun IrisApp(
         }
     }
     val state by controller.client.state.collectAsState()
+    val theme by controller.theme.collectAsState()
 
     IrisTheme {
-        Surface(modifier = Modifier.background(IrisColors.background)) {
-            val s = state
-            when (s) {
-                GatewayClient.State.Disconnected ->
-                    ConnectScreen(controller, prefillUrl = store.serverUrl, prefillToken = store.token)
-                is GatewayClient.State.AuthFailed ->
-                    ConnectScreen(
-                        controller,
-                        prefillUrl = store.serverUrl,
-                        prefillToken = store.token,
-                        initialError = "Pairing rejected: ${s.message}",
-                    )
-                // Connecting / Reconnecting / Connected all render the chat; the header
-                // StatusChip + connection banner show the link state without
-                // blocking the view (M7's full-screen spinner is gone).
-                else -> ChatScreen(controller)
+        CompositionLocalProvider(LocalUserTheme provides theme) {
+            // The color goes through Surface's `color` parameter: a
+            // Modifier.background on the Surface would be painted *under* the
+            // Surface's own colorScheme.surface fill and never show through.
+            // contentColor is pinned to the app's light text color — Surface
+            // would otherwise derive it from the user's background (black
+            // text on a light background), which would also darken text that
+            // sits on the app's own dark panels.
+            Surface(
+                modifier = Modifier.fillMaxSize(),
+                color = theme.background,
+                contentColor = IrisColors.onBackground,
+            ) {
+                val bgImage = rememberBackgroundImage(theme)
+                Box(modifier = Modifier.fillMaxSize()) {
+                    val bmp = bgImage
+                    if (bmp != null) {
+                        Image(
+                            bitmap = bmp,
+                            contentDescription = null,
+                            contentScale = ContentScale.Crop,
+                            modifier = Modifier.fillMaxSize(),
+                        )
+                    }
+                    val s = state
+                    when (s) {
+                        GatewayClient.State.Disconnected ->
+                            ConnectScreen(controller, prefillUrl = store.serverUrl, prefillToken = store.token)
+                        is GatewayClient.State.AuthFailed ->
+                            ConnectScreen(
+                                controller,
+                                prefillUrl = store.serverUrl,
+                                prefillToken = store.token,
+                                initialError = "Pairing rejected: ${s.message}",
+                            )
+                        // Connecting / Reconnecting / Connected all render the chat; the header
+                        // StatusChip + connection banner show the link state without
+                        // blocking the view (M7's full-screen spinner is gone).
+                        else -> ChatScreen(controller)
+                    }
+                }
             }
         }
     }