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/utils.ts

@@ -0,0 +1,78 @@
+import { generateMessageId } from "@/types/stream";
+
+export function generateAttachmentId(): string {
+  return `att_${generateMessageId()}`;
+}
+
+export function normalizeMimeType(input: string | undefined | null): string {
+  if (!input) {
+    return "image/jpeg";
+  }
+  const trimmed = input.trim();
+  return trimmed.length > 0 ? trimmed : "image/jpeg";
+}
+
+export function parseDataUrl(dataUrl: string): { mimeType: string; base64: string } {
+  const match = /^data:([^;,]+)?;base64,(.+)$/i.exec(dataUrl);
+  if (!match) {
+    throw new Error("Malformed data URL for attachment.");
+  }
+  const [, mimeTypeRaw, base64] = match;
+  if (!base64) {
+    throw new Error("Attachment data URL is missing base64 payload.");
+  }
+  return {
+    mimeType: normalizeMimeType(mimeTypeRaw),
+    base64,
+  };
+}
+
+export async function blobToBase64(blob: Blob): Promise<string> {
+  return await new Promise<string>((resolve, reject) => {
+    const reader = new FileReader();
+    reader.onload = () => {
+      if (typeof reader.result !== "string") {
+        reject(new Error("Unexpected FileReader result while encoding attachment."));
+        return;
+      }
+      const payload = reader.result.split(",", 2)[1];
+      if (!payload) {
+        reject(new Error("Attachment FileReader result did not contain base64 payload."));
+        return;
+      }
+      resolve(payload);
+    };
+    reader.onerror = () => {
+      reject(reader.error ?? new Error("Failed to read attachment blob."));
+    };
+    reader.readAsDataURL(blob);
+  });
+}
+
+export function pathToFileUri(path: string): string {
+  if (path.startsWith("file://")) {
+    return path;
+  }
+  if (path.startsWith("/")) {
+    return `file://${path}`;
+  }
+  return path;
+}
+
+export function fileUriToPath(uri: string): string {
+  if (!uri.startsWith("file://")) {
+    return uri;
+  }
+  return decodeURIComponent(uri.replace(/^file:\/\//, ""));
+}
+
+export function getFileExtensionFromName(fileName: string | null | undefined): string {
+  if (!fileName) {
+    return "";
+  }
+  const idx = fileName.lastIndexOf(".");
+  if (idx <= 0 || idx === fileName.length - 1) {
+    return "";
+  }
+  return fileName.slice(idx);
+}