Improve attachment memory safety, downloads, and high-memory alert UX.

Stream large receives to disk with chunk acks to cap renderer RAM, evict
off-screen display blobs, and route exports through a disk-aware download
service. Fix the high-memory dialog (backdrop dismiss, copy, log actions),
allow diagnostics paths in the path jail, and restore persisted image
hydration after reload.

Co-authored-by: Cursor <cursoragent@cursor.com>

toju-app/src/app/domains/attachment/domain/logic/attachment-blob.rules.ts

@@ -29,6 +29,13 @@ export function encodeUint8ArrayToBase64(bytes: Uint8Array): string {
   return btoa(binary);
 }
 
+/** Returns the decoded byte length of a base64 payload without allocating the bytes. */
+export function base64DecodedByteLength(base64: string): number {
+  const padding = base64.endsWith('==') ? 2 : base64.endsWith('=') ? 1 : 0;
+
+  return Math.max(0, Math.floor((base64.length * 3) / 4) - padding);
+}
+
 /** Yield control back to the browser so long attachment hydration cannot freeze Electron. */
 export function yieldToAttachmentHydrationLoop(): Promise<void> {
   return new Promise((resolve) => {