feat: implement universal icon system with Phosphor Icons

- Install @phosphor-icons/react package
- Create centralized Icon component with size/weight/color variants
- Create icon registry with 34 icons across 5 categories
- Replace all raw Unicode symbols with proper icon components
- Add icons to navigation, buttons, status indicators, git operations
- Add icon CSS with consistent sizing and spacing
- Fix type definitions for Phosphor icon compatibility

Quality gates: typecheck ✓, lint ✓, build ✓ (375KB bundle)

apps/web/src/components/syntax-highlighter.tsx

@@ -1,4 +1,6 @@
 import React, { useEffect, useState } from "react";
+
+import { Icon } from "./icon";
 import { highlightCode, loadLanguage } from "../utils/language";
 
 interface SyntaxHighlighterProps {
@@ -40,7 +42,17 @@ export const SyntaxHighlighter: React.FC<SyntaxHighlighterProps> = ({
           onClick={handleCopy}
           type="button"
         >
-          {copied ? "Copied!" : "Copy"}
+          {copied ? (
+            <>
+              <Icon name="success" size="sm" />
+              Copied!
+            </>
+          ) : (
+            <>
+              <Icon name="copy" size="sm" />
+              Copy
+            </>
+          )}
         </button>
       </div>
       <div className="code-container">