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/git-history.tsx

@@ -2,6 +2,7 @@ import { useCallback, useEffect, useState } from "react";
 import { useNavigate, useParams } from "react-router-dom";
 
 import { getCommitDetail, getRepositoryHistory, type CommitDetail, type CommitHistoryEntry } from "../api/git_repositories";
+import { Icon } from "../components/icon";
 
 export const GitHistoryPage = () => {
   const { projectId, repoId } = useParams<{ projectId: string; repoId: string }>();
@@ -70,6 +71,7 @@ export const GitHistoryPage = () => {
       <section className="stack">
         <p>Failed to load commit history</p>
         <button className="secondary-button" onClick={() => void loadHistory()} type="button">
+          <Icon name="refresh" size="sm" />
           Retry
         </button>
       </section>
@@ -155,7 +157,7 @@ export const GitHistoryPage = () => {
             <div className="detail-header">
               <h3>Commit Details</h3>
               <button className="ghost-button" onClick={handleCloseDetail} type="button">
-                ×
+                <Icon name="close" size="sm" />
               </button>
             </div>