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/file-editor.tsx

@@ -4,6 +4,7 @@ import { apiClient } from "../api/client";
 import { useAuth } from "../state/auth";
 import { CodeEditor } from "../components/code-editor";
 import { CommitDialog } from "../components/commit-dialog";
+import { Icon } from "../components/icon";
 import { SyntaxHighlighter } from "../components/syntax-highlighter";
 import { detectLanguage } from "../utils/language";
 
@@ -172,6 +173,7 @@ export const FileEditor: React.FC<FileEditorProps> = ({
               onClick={handleEdit}
               type="button"
             >
+              <Icon name="edit" size="sm" />
               Edit
             </button>
           )}
@@ -183,13 +185,24 @@ export const FileEditor: React.FC<FileEditorProps> = ({
             disabled={content === originalContent || saving}
             type="button"
           >
-            {saving ? "Saving..." : "Save"}
+            {saving ? (
+              <>
+                <Icon name="loading" size="sm" />
+                Saving...
+              </>
+            ) : (
+              <>
+                <Icon name="save" size="sm" />
+                Save
+              </>
+            )}
           </button>
               <button
                 className="btn-secondary"
                 onClick={handleCancel}
                 type="button"
               >
+                <Icon name="cancel" size="sm" />
                 Cancel
               </button>
             </>