Prepare for open source release under GPLv3

- Add GPLv3 LICENSE file
- Add CONTRIBUTING.md with guidelines
- Update README: remove Play Store section, add license and contributing refs
- Fix feature graphic (remove grid lines, replace Waybar chip with No root)

assets/feature-graphic.svg

@@ -1,13 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1024 500">
   <rect width="1024" height="500" fill="#ffffff"/>
 
-  <!-- Subtle grid -->
-  <g stroke="#e2e8f0" stroke-width="1">
-    <line x1="0" y1="125" x2="1024" y2="125"/>
-    <line x1="0" y1="250" x2="1024" y2="250"/>
-    <line x1="0" y1="375" x2="1024" y2="375"/>
-  </g>
-
   <!-- Phone - left -->
   <g transform="translate(100, 160)">
     <rect width="70" height="130" rx="12" fill="#1e293b"/>
@@ -46,13 +39,13 @@
       <text x="42" y="21" text-anchor="middle">REST API</text>
     </g>
     <g transform="translate(695, 310)">
-      <rect width="75" height="32" rx="16" fill="#f1f5f9" stroke="#e2e8f0"/>
-      <text x="37" y="21" text-anchor="middle">Waybar</text>
-    </g>
-    <g transform="translate(780, 310)">
       <rect width="95" height="32" rx="16" fill="#f1f5f9" stroke="#e2e8f0"/>
       <text x="47" y="21" text-anchor="middle">Auto-start</text>
     </g>
+    <g transform="translate(800, 310)">
+      <rect width="80" height="32" rx="16" fill="#f1f5f9" stroke="#e2e8f0"/>
+      <text x="40" y="21" text-anchor="middle">No root</text>
+    </g>
   </g>
 
   <!-- Code snippet -->