fix(ui): dither gradients to kill 8-bit sRGB banding on orb + mesh

app/src/main/java/dev/kaizen/app/chat/Background.kt

@@ -36,6 +36,7 @@ import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.BlurredEdgeTreatment
 import androidx.compose.ui.draw.blur
+import androidx.compose.ui.draw.drawWithContent
 import androidx.compose.ui.draw.scale
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.graphics.BlendMode
@@ -51,6 +52,8 @@ import dev.kaizen.app.ui.theme.AmberDeep
 import dev.kaizen.app.ui.theme.BlobAmber
 import dev.kaizen.app.ui.theme.BlobBlue
 import dev.kaizen.app.ui.theme.BlobTeal
+import dev.kaizen.app.ui.theme.DITHER_ALPHA
+import dev.kaizen.app.ui.theme.rememberDitherBrush
 
 /** Mesh/blob background: large, heavily blurred color circles that drift slowly. */
 @Composable
@@ -66,7 +69,17 @@ fun MeshBackground(content: @Composable BoxScope.() -> Unit) {
         animationSpec = infiniteRepeatable(tween(22000, easing = LinearEasing), RepeatMode.Reverse),
         label = "drift",
     )
-    Box(Modifier.fillMaxSize().background(base)) {
+    // Dither overlay (drawn last, over everything) — kills 8-bit gradient banding.
+    val ditherBrush = rememberDitherBrush()
+    Box(
+        Modifier
+            .fillMaxSize()
+            .background(base)
+            .drawWithContent {
+                drawContent()
+                drawRect(brush = ditherBrush, alpha = DITHER_ALPHA)
+            },
+    ) {
         Blob(BlobAmber, 360.dp, (-60f + drift * 30f).dp, (-40f + drift * 24f).dp, 0.30f * factor)
         Blob(BlobBlue, 380.dp, (170f - drift * 44f).dp, (150f + drift * 40f).dp, 0.24f * factor)
         Blob(BlobTeal, 300.dp, (30f + drift * 50f).dp, (560f - drift * 36f).dp, 0.22f * factor)

app/src/main/java/dev/kaizen/app/ui/theme/Dither.kt

@@ -0,0 +1,52 @@
+package dev.kaizen.app.ui.theme
+
+import android.graphics.Bitmap
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.graphics.ImageBitmap
+import androidx.compose.ui.graphics.ImageShader
+import androidx.compose.ui.graphics.ShaderBrush
+import androidx.compose.ui.graphics.TileMode
+import androidx.compose.ui.graphics.asImageBitmap
+import kotlin.random.Random
+
+/**
+ * Cross-version gradient dither.
+ *
+ * 8-bit sRGB radial gradients quantize into visible concentric rings (the
+ * "verpickelt"/banding look on the orb and mesh blobs). Overlaying a tiled,
+ * ~1-LSB black/white/transparent noise pattern trades those hard bands for
+ * imperceptible fine grain — the standard fix that works on every API level
+ * (no AGSL / API-33 gating needed, so it covers minSdk 26).
+ *
+ * The pattern is deterministic (fixed seed) so it never shimmers between
+ * recompositions; ~⅓ black, ⅓ white, ⅓ transparent makes the perturbation
+ * symmetric (darkens and brightens equally) at a low overall [DITHER_ALPHA].
+ *
+ * [DITHER_ALPHA] is the single tuning knob: higher = bands vanish faster but
+ * fine grain creeps in (worst on the near-black Obsidian background); lower =
+ * cleaner but bands may survive. ~0.012–0.018 is the sweet spot — dial on device.
+ */
+const val DITHER_ALPHA = 0.015f
+
+private fun buildNoiseBitmap(size: Int = 160): ImageBitmap {
+    val pixels = IntArray(size * size)
+    val rnd = Random(0x4A1B)
+    for (i in pixels.indices) {
+        pixels[i] = when (rnd.nextInt(3)) {
+            0 -> 0xFF000000.toInt() // darken
+            1 -> 0xFFFFFFFF.toInt() // brighten
+            else -> 0x00000000      // leave untouched
+        }
+    }
+    val bmp = Bitmap.createBitmap(size, size, Bitmap.Config.ARGB_8888)
+    bmp.setPixels(pixels, 0, size, 0, 0, size, size)
+    return bmp.asImageBitmap()
+}
+
+/** A tiled noise [ShaderBrush] for dithering. Build once, fill the whole surface at [DITHER_ALPHA]. */
+@Composable
+fun rememberDitherBrush(): ShaderBrush {
+    val image = remember { buildNoiseBitmap() }
+    return remember(image) { ShaderBrush(ImageShader(image, TileMode.Repeated, TileMode.Repeated)) }
+}