feat: v0.2.0 - add system tray, clean up unused role field

- Replace text '+' icon with centered SVG plus icon for the add-provider button
- Add system tray: minimize-to-tray and quit-from-tray menu (minimize hides window, close exits app)
- Remove unused model role field (no functional effect, only stored/displayed)
- Fix multiple providers showing as active in Kimi Code mode: only the default_model's provider is marked active
- Copy /reload to clipboard after provider switch as a convenience hint
- Bump version to 0.2.0

public/kimi.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#3b82f6"/>
+      <stop offset="100%" stop-color="#8b5cf6"/>
+    </linearGradient>
+  </defs>
+  <rect width="100" height="100" rx="22" fill="url(#g)"/>
+  <text x="48" y="70" text-anchor="middle" font-size="52" font-weight="700"
+        fill="white" font-family="system-ui, -apple-system, 'Segoe UI', sans-serif">K</text>
+  <circle cx="74" cy="26" r="9" fill="white" opacity="0.35"/>
+  <circle cx="74" cy="26" r="6.5" fill="white" opacity="0.92"/>
+</svg>