fix(email): avoid undefined in subjectless replies and forwards

[Whxuan0701]

Summary

Replying to or forwarding an email without a subject currently produces the literal subjects Re: undefined and Fwd: undefined.

Current implementation

apps/web/src/features/block-email/util/subjectText.ts interpolates replyingTo.subject directly for both paths. ApiMessage.subject is optional, so a subjectless message reaches the template literal as undefined.

Expected result

• Reply and Reply All use Re: for a subjectless message.
• Forward uses Fwd: for a subjectless message.
• Existing subjects and existing case-insensitive Re: prefixes keep their current behavior.

Suggested direction

Normalize the optional source subject before adding the reply or forward prefix, and add focused tests in subjectText.test.ts.

Non-goals

• Changing how subjects are displayed outside the composer.
• Changing provider-specific or localized subject prefixes.