chore(icons): refresh app icon set + add android/ios variants and source

Regenerate desktop icons (png/ico/icns), add 64x64 and mobile (android mipmap,
ios AppIcon) variants, and keep the editable icon-source.svg/png.

src-tauri/icons/android/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+  <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <background android:drawable="@color/ic_launcher_background"/>
+</adaptive-icon>

src-tauri/icons/android/values/ic_launcher_background.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+  <color name="ic_launcher_background">#fff</color>
+</resources>

src-tauri/icons/icon-source.svg

@@ -0,0 +1,39 @@
+<svg width="1024" height="1024" viewBox="0 0 1024 1024" xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#27406e"/>
+      <stop offset="0.55" stop-color="#172f4f"/>
+      <stop offset="1" stop-color="#0d1730"/>
+    </linearGradient>
+    <radialGradient id="glow" cx="0.5" cy="0.42" r="0.6">
+      <stop offset="0" stop-color="#3a5fa0" stop-opacity="0.55"/>
+      <stop offset="1" stop-color="#3a5fa0" stop-opacity="0"/>
+    </radialGradient>
+    <linearGradient id="ivory" x1="0.2" y1="0.05" x2="0.85" y2="0.95">
+      <stop offset="0" stop-color="#fffdf6"/>
+      <stop offset="0.55" stop-color="#f3e8d2"/>
+      <stop offset="1" stop-color="#d8c39a"/>
+    </linearGradient>
+    <filter id="ds" x="-20%" y="-20%" width="140%" height="140%">
+      <feDropShadow dx="0" dy="14" stdDeviation="18" flood-color="#000000" flood-opacity="0.35"/>
+    </filter>
+  </defs>
+
+  <rect x="0" y="0" width="1024" height="1024" rx="224" fill="url(#bg)"/>
+  <rect x="0" y="0" width="1024" height="1024" rx="224" fill="url(#glow)"/>
+
+  <g filter="url(#ds)">
+    <!-- single bold tusk: thick rounded root at upper-right, tapering along a
+         gentle curve to a sharp tip at lower-left -->
+    <path d="M 712 408
+             C 720 600, 560 720, 322 742
+             C 470 700, 600 470, 628 318
+             A 60 60 0 0 1 712 408 Z"
+          fill="url(#ivory)"/>
+    <!-- soft sheen along the belly (upper-left edge) for ivory depth -->
+    <path d="M 322 742
+             C 470 700, 600 470, 628 318
+             C 612 470, 470 660, 360 726 Z"
+          fill="#ffffff" opacity="0.18"/>
+  </g>
+</svg>