Addresses dev-test feedback:
- Sidebar + reading-pane addresses (rechnungen@ / hello@ / no-reply@) are now
read from the mail config via GET /admin/email/identities, not hardcoded.
- Highlight/selection now uses the branding accent (bg-accent-soft /
text-on-accent-soft / accent-dark) instead of hardcoded blue, so it follows
the admin's CI colour like the sidebar.
- Header gains "New message" (compose) and "Sync" (poll mailboxes now) buttons.
- Composer modal enlarged (920px, taller editable body).
- Customer mailbox (hello@) now has BOTH incoming (IMAP) and outgoing (SMTP)
settings — migration 156 adds smtp_* + from_* to mail_accounts;
emailProcessor.sendRawEmail takes an accountKey and sends via that mailbox's
SMTP identity (falls back to the global from). Manual/reply sends from the
Messages UI use the 'customers' identity, so replies come from hello@.
Frontend build + migration boot (156) verified.