Custom domains & branding for outgoing links (client portal, proposals, invoices)

Contractors want to white-label outgoing communications so that SMS messages, client portal links, proposals, and invoices appear to come from their own domain/brand — not from Handoff. Currently all outgoing links show app.handoff.ai and SMS previews show the Handoff brand.


**Proposed solution:**
- Allow contractors to configure a custom domain for outgoing links
- Update OG/meta tags on client portal links to reflect the contractor's company name and branding instead of Handoff
- Cover all outgoing surfaces: client portal invite links, proposal links, invoice links, SMS notifications