feat(FN-3528): add line-number gutter toggle to FileEditor and Files modal

This merge adds a line-number toggle to the Files modal with persisted preference, introduces an AgentAvatar component with server-side avatar storage routes, and adds documentation for both the toggle and avatar storage. It also includes a regression test for runtime plugin alias behavior and updat

Fusion-Task-Id: FN-3528

packages/dashboard/app/utils/__tests__/projectStorage.test.ts

@@ -93,9 +93,10 @@ describe("projectStorage", () => {
         "kb-usage-modal-size",
         "kb-usage-provider-order",
         "kb-chat-active-session",
+        "kb-files-line-numbers",
       ]),
     );
-    expect(PROJECT_STORAGE_KEYS).toHaveLength(19);
+    expect(PROJECT_STORAGE_KEYS).toHaveLength(20);
   });
 
   it("has no overlap between global and project-scoped keys", () => {