refactor(app): introduce attachment storage layer and draft lifecycle management

Replace inline data-URL image handling with a persistent attachment storage service, add draft lifecycle states (active/abandoned/sent) with generation tracking, and update fetchAgent to return project placement alongside agent snapshots.

packages/app/src/stores/draft-keys.ts

@@ -0,0 +1,18 @@
+import { generateMessageId } from "@/types/stream";
+
+export function generateDraftId(): string {
+  return `draft_${generateMessageId()}`;
+}
+
+export function buildDraftStoreKey(input: {
+  serverId: string;
+  agentId: string;
+  draftId?: string | null;
+}): string {
+  const serverId = input.serverId.trim();
+  const explicitDraftId = input.draftId?.trim();
+  if (explicitDraftId) {
+    return `draft:${serverId}:${explicitDraftId}`;
+  }
+  return `agent:${serverId}:${input.agentId.trim()}`;
+}