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/profile.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 
 import { getProfile, updateProfile, uploadAvatar } from "../api/profile";
+import { Icon } from "../components/icon";
 import { useAuth } from "../state/auth";
 import type { UserProfile } from "../api/profile";
 
@@ -99,6 +100,7 @@ export const ProfilePage = () => {
         <div className="card stack">
           <p>Failed to load profile</p>
           <button className="secondary-button" onClick={() => void loadProfile()} type="button">
+            <Icon name="refresh" size="sm" />
             Retry
           </button>
         </div>
@@ -120,7 +122,17 @@ export const ProfilePage = () => {
               onClick={() => fileInputRef.current?.click()}
               type="button"
             >
-              {status === "saving" ? "Uploading..." : "Change Avatar"}
+              {status === "saving" ? (
+                <>
+                  <Icon name="loading" size="sm" />
+                  Uploading...
+                </>
+              ) : (
+                <>
+                  <Icon name="edit" size="sm" />
+                  Change Avatar
+                </>
+              )}
             </button>
             <input
               accept="image/png,image/jpeg"
@@ -162,7 +174,17 @@ export const ProfilePage = () => {
               onClick={() => void handleSave()}
               type="button"
             >
-              {status === "saving" ? "Saving..." : "Save Changes"}
+              {status === "saving" ? (
+                <>
+                  <Icon name="loading" size="sm" />
+                  Saving...
+                </>
+              ) : (
+                <>
+                  <Icon name="save" size="sm" />
+                  Save Changes
+                </>
+              )}
             </button>
           </div>
         </div>