feat: Auto-generate email subject when composing a new email

[Copilot]

When composing a new email, the AI only generated the body — the subject field had to be filled manually. This adds automatic subject generation when the subject field is empty.

Changes

OpenAI provider

• Prompt now requests subject generation when no subject is provided, using Subject: [subject here] as a format marker at the top of the response
• Response parser extracts and strips the Subject: line, setting it separately on the Respond object

GenereteEmailAction

• JSON response now includes a subject field alongside the existing respond

Frontend (sendPostRequest.js / bundle)

• On successful generation, if the response includes a subject and compose-subject is empty, the field is auto-filled
• Existing subjects (replies, already-generated) are never overwritten

Tests

• Updated all prompt string assertions to match the new format
• Added testGenerateEmailParsesSubjectFromResponse — subject extracted, body cleaned
• Added testGenerateEmailNoSubjectInResponse — null subject when AI omits the line

Prompt format change

// Before (no subject case):
"... Without a subject ..."
The format should be as follows:
Greeting
...

// After:
"... Generate a suitable subject for this email. ..."
The format should be as follows:
Subject: [subject here]

Greeting
...