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/dashboard.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useState } from "react";
 
 import { getDashboardSummary, type DashboardSummary } from "../api/dashboard";
+import { Icon } from "../components/icon";
 
 const CARDS = [
   { label: "Projects", key: "projects" },
@@ -50,6 +51,7 @@ export const DashboardPage = () => {
         <div className="card stack">
           <p>Dashboard is unavailable</p>
           <button className="secondary-button" onClick={() => void loadSummary()} type="button">
+            <Icon name="refresh" size="sm" />
             Retry
           </button>
         </div>
@@ -68,9 +70,11 @@ export const DashboardPage = () => {
 
       <div className="quick-actions">
         <button className="primary-button" type="button">
+          <Icon name="add" size="sm" />
           New Project
         </button>
         <button className="secondary-button" type="button">
+          <Icon name="add" size="sm" />
           Add Repository
         </button>
       </div>