[P0.3] Template email authentication and redirect configuration

### Goal
Configure Gmail SMTP for Supabase auth emails, email confirmation and recovery settings, app redirect URLs, and branded email templates.

### Dependencies
[P0.1], [P0.2]

### Acceptance Criteria
- SMTP credentials configured
- Redirect URLs registered in Supabase Auth
- Branded email templates installed

### Run
`/setup-auth`

Start by reviewing the `/setup-auth` entry point and the work from dependencies [P0.1] and [P0.2]. Configure Gmail SMTP, register the app redirect URLs in Supabase Auth, and install the branded email templates; done means all three acceptance criteria are met.

Written by the indexing model from the issue text.