fix: honor the user's filename template for forward-as-attachment

buildForwardAsAttachmentPayload called emailExportFilename(email) with
no options, always using the default naming template regardless of
the user's configured emailDownloadTemplate (and space/case/diacritics
transforms) - the same settings the neighboring "Export as .eml"
action already respects. That could produce inconsistent .eml
filenames between the two actions for the same message.

Accept an optional EmailFilenameOptions parameter and pass it through.
handleForwardAsAttachment now reads the same settings
email-viewer.tsx's emailFilenameOptions useMemo does, via
useSettingsStore.getState() (a one-off read inside an event handler,
matching this file's existing pattern, rather than a new reactive
subscription). Add a unit test covering a custom template.

Caught by GitHub Copilot's automated PR review.

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

@@ -45,4 +45,14 @@ describe('buildForwardAsAttachmentPayload', () => {
     const payload = buildForwardAsAttachmentPayload(email, 'Fwd:');
     expect(payload?.subject).toBe('Fwd: already forwarded once');
   });
+
+  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:', {
+      template: 'custom-{subject}',
+      lowercase: true,
+      spaceReplacement: 'dash',
+    });
+    expect(payload?.attachment.name).toBe('custom-missed-spam-example.eml');
+  });
 });