feat(desktop): wrap logo in macOS squircle so dock icon looks native

macOS Big Sur+ mandates the squircle icon template for dock / Cmd+Tab —
bare transparent logos stick out next to first-party apps. PlatformAppIcon
wraps the source logo in a 1024x1024 white squircle with 10% padding at
runtime, only when the host OS is macOS (PlatformInfo.host, not .current
— the override is for in-app theming, the dock is drawn by the real OS).

No extra image file: Java2D renders the squircle from the existing
icon.png so Linux / Windows paths stay untouched (they still get the
raw transparent logo, which is the right call for GNOME and matches
Windows 11 taskbar rendering).

Apple's true shape is a superellipse; Java2D lacks a primitive for it,
so RoundRectangle2D with 22.5% corner radius is the practical approx
(invisible difference at dock icon sizes). Applied to both
java.awt.Taskbar.iconImage (dock) and Window(icon=) (title-bar thumb).

https://claude.ai/code/session_01NufduPfZvYQVYwLkbCjCUo

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/Main.kt

@@ -202,14 +202,21 @@ fun main() {
     // iconImage, NOT the Window(icon=) composable parameter (which only sets
     // the in-title-bar proxy icon). Without this, a JVM launched via gradle
     // shows the generic Java coffee-cup square in Cmd+Tab. ImageIO preserves
-    // the PNG alpha channel so the dock renders the logo with transparency.
+    // the PNG alpha channel so the dock renders the logo with transparency;
+    // on macOS the logo is then wrapped in a squircle so it matches
+    // first-party dock icons.
     try {
         val bytes = Unit::class.java.getResourceAsStream("/icon.png")!!.readBytes()
-        val awtImage = javax.imageio.ImageIO.read(java.io.ByteArrayInputStream(bytes))
+        val raw = javax.imageio.ImageIO.read(java.io.ByteArrayInputStream(bytes))
-        if (awtImage != null && java.awt.Taskbar.isTaskbarSupported()) {
+        val adapted =
+            raw?.let {
+                com.vitorpamplona.amethyst.desktop.platform.PlatformAppIcon
+                    .adaptForHost(it)
+            }
+        if (adapted != null && java.awt.Taskbar.isTaskbarSupported()) {
             val taskbar = java.awt.Taskbar.getTaskbar()
             if (taskbar.isSupported(java.awt.Taskbar.Feature.ICON_IMAGE)) {
-                taskbar.iconImage = awtImage
+                taskbar.iconImage = adapted
             }
         }
     } catch (e: Exception) {
@@ -274,15 +281,21 @@ fun main() {
         // Callback set by App() for single pane navigation from MenuBar
         var navigateToScreen by remember { mutableStateOf<((DeckColumnType) -> Unit)?>(null) }
 
-        // Transparent 512x512 PNG shown in the macOS dock / Windows taskbar / GNOME
+        // Window title-bar / taskbar thumbnail icon. On macOS the source logo
-        // & KDE task switchers while running via gradle (the packaged app uses the
+        // is wrapped in a squircle so it matches every other dock icon; on
-        // icon.icns / icon.ico configured in nativeDistributions).
+        // other platforms the raw transparent logo is used as-is.
         val appIcon =
             remember {
                 val bytes = Unit::class.java.getResourceAsStream("/icon.png")!!.readBytes()
+                val raw = javax.imageio.ImageIO.read(java.io.ByteArrayInputStream(bytes))
+                val adapted =
+                    com.vitorpamplona.amethyst.desktop.platform.PlatformAppIcon
+                        .adaptForHost(raw)
+                val buf = java.io.ByteArrayOutputStream()
+                javax.imageio.ImageIO.write(adapted, "png", buf)
                 val bitmap =
                     org.jetbrains.skia.Image
-                        .makeFromEncoded(bytes)
+                        .makeFromEncoded(buf.toByteArray())
                         .toComposeImageBitmap()
                 BitmapPainter(bitmap)
             }

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/platform/PlatformAppIcon.kt

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.desktop.platform
+
+import java.awt.AlphaComposite
+import java.awt.Color
+import java.awt.RenderingHints
+import java.awt.geom.RoundRectangle2D
+import java.awt.image.BufferedImage
+
+/**
+ * Adapts a transparent source logo to the host OS's app-icon conventions.
+ *
+ * - macOS (Big Sur+) requires a "squircle" app icon template: the mark sits on
+ *   a rounded-square background at ~22.5% corner radius with padding around the
+ *   edges. A raw transparent logo in the dock looks out-of-place next to
+ *   first-party apps, so we wrap it in a white squircle when running on macOS.
+ * - Other platforms return the source image unchanged — Windows and most Linux
+ *   DEs render transparent icons fine, and GNOME apps specifically tend to
+ *   avoid background shapes.
+ */
+object PlatformAppIcon {
+    /**
+     * Wraps [source] in a macOS-style squircle if the host OS is macOS; returns
+     * the source unchanged otherwise. Not scaled by [PlatformInfo.current] (the
+     * preview override) — the dock is drawn by the real host OS, so only the
+     * real host platform matters here.
+     */
+    fun adaptForHost(source: BufferedImage): BufferedImage =
+        when (PlatformInfo.host) {
+            Platform.MACOS -> macOsSquircle(source)
+            else -> source
+        }
+
+    /**
+     * Renders [source] inside a 1024x1024 white squircle with padding around
+     * the mark. Matches Apple's HIG macOS app icon template (Big Sur+).
+     *
+     * Implementation notes:
+     * - Apple's "squircle" is a superellipse, not a standard rounded rectangle;
+     *   Java2D doesn't ship a superellipse primitive, so [RoundRectangle2D]
+     *   with a 22.5% corner radius is the practical approximation used by most
+     *   third-party tooling. At dock-icon sizes the difference is invisible.
+     * - 10% padding on each side keeps the mark from touching the squircle
+     *   corners, matching Apple's template content area (824/1024 ≈ 80%).
+     */
+    private fun macOsSquircle(source: BufferedImage): BufferedImage {
+        val size = 1024
+        val cornerDiameter = (size * 0.45f) // diameter = 2 * radius; radius = 22.5%
+        val padding = (size * 0.10f).toInt()
+        val innerSize = size - 2 * padding
+
+        val out = BufferedImage(size, size, BufferedImage.TYPE_INT_ARGB)
+        val g = out.createGraphics()
+        try {
+            g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON)
+            g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC)
+            g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY)
+
+            // Paint the squircle background in white. Clip must be set BEFORE
+            // the fill so the fill respects the rounded corners.
+            val squircle =
+                RoundRectangle2D.Float(
+                    0f,
+                    0f,
+                    size.toFloat(),
+                    size.toFloat(),
+                    cornerDiameter,
+                    cornerDiameter,
+                )
+            g.composite = AlphaComposite.Src
+            g.color = Color(0xFFFFFF)
+            g.fill(squircle)
+
+            // Composite the source logo on top, scaled into the padded area.
+            g.composite = AlphaComposite.SrcOver
+            g.clip = squircle // keep the logo within the squircle on overhang
+            g.drawImage(source, padding, padding, innerSize, innerSize, null)
+        } finally {
+            g.dispose()
+        }
+        return out
+    }
+}