style: simplify app icon branding

public/icons/maskable.svg

@@ -1,12 +1,7 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
-  <defs>
-    <linearGradient id="g" x1="40" y1="20" x2="480" y2="500" gradientUnits="userSpaceOnUse">
-      <stop stop-color="#38bdf8"/>
-      <stop offset=".55" stop-color="#2563eb"/>
-      <stop offset="1" stop-color="#312e81"/>
-    </linearGradient>
-  </defs>
-  <rect width="512" height="512" fill="url(#g)"/>
-  <circle cx="392" cy="124" r="90" fill="#fff" fill-opacity=".12"/>
-  <text x="62" y="318" fill="white" font-family="Arial, Helvetica, sans-serif" font-size="186" font-weight="700" letter-spacing="-28">wtr.</text>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="wtr.">
+  <rect width="512" height="512" fill="#171d25"/>
+  <rect x="44" y="44" width="424" height="424" rx="96" fill="#1f2630"/>
+  <rect x="68" y="68" width="376" height="376" rx="78" fill="none" stroke="#eef3f7" stroke-opacity=".14" stroke-width="2"/>
+  <text x="84" y="318" fill="#eef3f7" font-family="Arial, Helvetica, sans-serif" font-size="172" font-weight="700" letter-spacing="-26">wtr</text>
+  <circle cx="386" cy="296" r="17" fill="#8fb4ce"/>
 </svg>