Simpler setup and sign-in

Local setup is one command, and sign-in works without social credentials.

• Setting up Teak locally is now a single command that installs dependencies, provisions an isolated backend, and writes the local configuration.
• Signing in with email works without configuring Google or Apple first. The Google and Apple buttons stay visible and explain clearly when they are unavailable.
• If you self-host Teak with custom addresses for the API or the assistant integration, replace those settings with a single public origin as described in the self-hosting guide. Sign-in keeps using SITE_URL.