refactor(titlebar): replace hand-drawn restored-window SVG with Phosphor Copy icon

P3-56 (renderer): the maximized-state button used a custom 3-rect SVG to
render the 'restored window' glyph while the minimize and close buttons
used Phosphor icons. Swap the SVG for Phosphor's Copy icon so all three
titlebar buttons share the same icon family and weight, and the visual
bundle no longer carries a one-off raster-like SVG block.

src/renderer/components/TitleBar.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useCallback } from "react";
-import { Minus, Square, X } from "@phosphor-icons/react";
+import { Copy, Minus, Square, X } from "@phosphor-icons/react";
 
 export default function TitleBar() {
   const [maximized, setMaximized] = useState(false);
@@ -41,11 +41,7 @@ export default function TitleBar() {
           }}
         >
           {maximized ? (
-            <svg width="10" height="10" viewBox="0 0 10 10" fill="none" stroke="currentColor" strokeWidth="1.2">
-              <rect x="2" y="0" width="8" height="8" rx="1" />
-              <rect x="0" y="2" width="8" height="8" rx="1" fill="var(--card)" />
-              <rect x="0" y="2" width="8" height="8" rx="1" />
-            </svg>
+            <Copy size={12} weight="bold" />
           ) : (
             <Square size={12} weight="bold" />
           )}