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/attachments/store.ts

@@ -0,0 +1,38 @@
+import { Platform } from "react-native";
+import { isTauriEnvironment } from "@/utils/tauri";
+import type { AttachmentStore } from "@/attachments/types";
+
+let attachmentStorePromise: Promise<AttachmentStore> | null = null;
+
+async function createAttachmentStore(): Promise<AttachmentStore> {
+  if (Platform.OS === "web") {
+    if (isTauriEnvironment()) {
+      const { createDesktopAttachmentStore } = await import(
+        "@/desktop/attachments/desktop-attachment-store"
+      );
+      return createDesktopAttachmentStore();
+    }
+
+    const { createIndexedDbAttachmentStore } = await import(
+      "@/attachments/web/indexeddb-attachment-store"
+    );
+    return createIndexedDbAttachmentStore();
+  }
+
+  const { createNativeFileAttachmentStore } = await import(
+    "@/attachments/native/native-file-attachment-store"
+  );
+  return createNativeFileAttachmentStore();
+}
+
+export async function getAttachmentStore(): Promise<AttachmentStore> {
+  if (!attachmentStorePromise) {
+    attachmentStorePromise = createAttachmentStore();
+  }
+  return await attachmentStorePromise;
+}
+
+/** Test-only hook to inject a deterministic store implementation. */
+export function __setAttachmentStoreForTests(store: AttachmentStore | null): void {
+  attachmentStorePromise = store ? Promise.resolve(store) : null;
+}