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/commit-dialog.tsx

@@ -1,5 +1,7 @@
 import React, { useState } from "react";
 
+import { Icon } from "./icon";
+
 interface CommitDialogProps {
   isOpen: boolean;
   filePath: string;
@@ -129,6 +131,7 @@ export const CommitDialog: React.FC<CommitDialogProps> = ({
             onClick={onCancel}
             type="button"
           >
+            <Icon name="cancel" size="sm" />
             Cancel
           </button>
           <button
@@ -137,7 +140,17 @@ export const CommitDialog: React.FC<CommitDialogProps> = ({
             disabled={loading || !hasChanges || !message.trim()}
             type="button"
           >
-            {loading ? "Committing..." : "Commit Changes"}
+            {loading ? (
+              <>
+                <Icon name="loading" size="sm" />
+                Committing...
+              </>
+            ) : (
+              <>
+                <Icon name="commit" size="sm" />
+                Commit Changes
+              </>
+            )}
           </button>
         </div>
       </div>