feat(relay): Configure a separate background delivery relay#93

What Changed

Carries pingdotgg/t3code#16618 background-relay-configuration by jakeleventhal. Mobile background registration, device listing, Live Activities and activity snapshots can use an optional separate relay with its own token cache. Servers can override the background publish destination, credential and issuer while keeping the activity sharing switch and the existing T3 Connect configuration. Invalid overrides fail instead of falling back to the primary relay.

Why

The personal relay layers above need a background destination separate from T3 Connect. Drop this layer at the refresh after #16618 background-relay-configuration merges upstream.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes