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/pages/placeholder.tsx

@@ -18,6 +18,8 @@ export const NotFoundPage = () => {
 
 const API_BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
 
+import { Icon } from "../components/icon";
+
 export const LoginRedirectPage = () => {
   const nextPath = new URLSearchParams(window.location.search).get("next") ?? "/";
   const encodedNext = encodeURIComponent(nextPath);
@@ -27,6 +29,7 @@ export const LoginRedirectPage = () => {
       <h1>Sign in required</h1>
       <p className="muted">You need to authenticate to access this section.</p>
       <a className="primary-button" href={`${API_BASE_URL}/auth/login?next=${encodedNext}`}>
+        <Icon name="profile" size="sm" />
         Continue to login
       </a>
     </section>