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/__tests__/forward-as-attachment.test.ts

@@ -46,6 +46,12 @@ describe('buildForwardAsAttachmentPayload', () => {
     expect(payload?.subject).toBe('Fwd: already forwarded once');
   });
 
+  it('leaves the subject blank (not just the bare prefix) for a subject-less message, matching normal Forward', () => {
+    const email = makeEmail({ subject: undefined });
+    const payload = buildForwardAsAttachmentPayload(email, 'Fwd:');
+    expect(payload?.subject).toBe('');
+  });
+
   it('honors a custom filename template, matching "Export as .eml" naming instead of always using the default', () => {
     const email = makeEmail({ subject: 'Missed spam example' });
     const payload = buildForwardAsAttachmentPayload(email, 'Fwd:', {