fix: leave subject blank (not "Fwd:") for a subject-less message

buildForwardAsAttachmentPayload called buildForwardSubject(email.subject,
forwardPrefix) unconditionally, and buildForwardSubject("", prefix)
returns just the bare prefix rather than "". Normal Forward doesn't do
this - EmailComposer's getInitialSubject() returns "" outright when
!replyTo?.subject, only calling buildForwardSubject when there's an
actual subject to prefix. So forwarding a subject-less message as an
attachment produced "Fwd:" as the subject, while normal Forward left
it blank.

Only call buildForwardSubject when email.subject is truthy, matching
getInitialSubject()'s behavior exactly. Add a test.

Caught by GitHub Copilot's automated PR review.

lib/forward-as-attachment.ts

@@ -41,7 +41,11 @@ export function buildForwardAsAttachmentPayload(
   if (!email.blobId) return null;
 
   return {
-    subject: buildForwardSubject(email.subject, forwardPrefix),
+    // Match the normal Forward flow's getInitialSubject(), which leaves the
+    // subject blank rather than prefix-only when the original has none -
+    // buildForwardSubject("", prefix) would otherwise return just the bare
+    // prefix (e.g. "Fwd:") for a subject-less message.
+    subject: email.subject ? buildForwardSubject(email.subject, forwardPrefix) : "",
     attachment: {
       blobId: email.blobId,
       name: emailExportFilename(email, filenameOptions),