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/projectStorage.ts

@@ -26,6 +26,7 @@ export const PROJECT_STORAGE_KEYS: string[] = [
   "kb-usage-modal-size",
   "kb-usage-provider-order",
   "kb-chat-active-session",
+  "kb-files-line-numbers",
 ];
 
 export function scopedKey(baseKey: string, projectId?: string): string {