feat(android): add animated gradient border composable

android/app/src/main/java/com/thisux/droidclaw/overlay/GradientBorder.kt

@@ -0,0 +1,85 @@
+package com.thisux.droidclaw.overlay
+
+import androidx.compose.animation.core.LinearEasing
+import androidx.compose.animation.core.RepeatMode
+import androidx.compose.animation.core.animateFloat
+import androidx.compose.animation.core.infiniteRepeatable
+import androidx.compose.animation.core.rememberInfiniteTransition
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.Canvas
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Brush
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.unit.dp
+
+private val GradientColors = listOf(
+    Color(0xFF8B5CF6), // purple
+    Color(0xFF3B82F6), // blue
+    Color(0xFF06B6D4), // cyan
+    Color(0xFF10B981), // green
+    Color(0xFF8B5CF6), // purple (loop)
+)
+
+@Composable
+fun GradientBorder() {
+    val transition = rememberInfiniteTransition(label = "gradientRotation")
+    val offset by transition.animateFloat(
+        initialValue = 0f,
+        targetValue = 1f,
+        animationSpec = infiniteRepeatable(
+            animation = tween(durationMillis = 3000, easing = LinearEasing),
+            repeatMode = RepeatMode.Restart
+        ),
+        label = "gradientOffset"
+    )
+
+    val borderWidth = with(LocalDensity.current) { 4.dp.toPx() }
+
+    Canvas(modifier = Modifier.fillMaxSize()) {
+        val w = size.width
+        val h = size.height
+
+        val shiftedColors = shiftColors(GradientColors, offset)
+
+        // Top edge
+        drawRect(
+            brush = Brush.horizontalGradient(shiftedColors),
+            topLeft = Offset.Zero,
+            size = Size(w, borderWidth)
+        )
+
+        // Bottom edge
+        drawRect(
+            brush = Brush.horizontalGradient(shiftedColors.reversed()),
+            topLeft = Offset(0f, h - borderWidth),
+            size = Size(w, borderWidth)
+        )
+
+        // Left edge
+        drawRect(
+            brush = Brush.verticalGradient(shiftedColors),
+            topLeft = Offset.Zero,
+            size = Size(borderWidth, h)
+        )
+
+        // Right edge
+        drawRect(
+            brush = Brush.verticalGradient(shiftedColors.reversed()),
+            topLeft = Offset(w - borderWidth, 0f),
+            size = Size(borderWidth, h)
+        )
+    }
+}
+
+private fun shiftColors(colors: List<Color>, offset: Float): List<Color> {
+    if (colors.size < 2) return colors
+    val n = colors.size
+    val shift = (offset * n).toInt() % n
+    return colors.subList(shift, n) + colors.subList(0, shift)
+}