## Problem
Emails sent via send_gmail_message were always showing as from a default
user account regardless of which account was specified in the
user_google_email parameter.
## Root Cause
The _prepare_gmail_message() function was not setting the From header in
the MIME message. Gmail API defaults to the primary send-as address when
no From header is provided.
## Solution
- Added from_email parameter to _prepare_gmail_message()
- Set From header explicitly: message["From"] = from_email
- Updated send_gmail_message() to pass user_google_email as from_email
- Updated draft_gmail_message() to pass user_google_email as from_email
- Capitalized MIME headers (Subject, From, To, Cc, Bcc) per conventions
## Testing
Verified emails now correctly show sender address for multiple accounts.
Each account now properly displays its own address in the From field.
Fixes multi-account email sending functionality.
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