[Mobile] Add release-channel configuration contract tests

## Problem
Development, staging, and production configurations can drift across Expo, API, Stellar network, Sentry, and update channels, creating a risk of shipping a build pointed at the wrong environment.

## Scope
Define one typed configuration contract and test every release profile against it.

## Acceptance criteria
- Each profile declares API base URL, Stellar network, asset identifiers, update channel, and telemetry destination as one validated unit.
- Production builds fail validation for testnet URLs, placeholder values, or missing required secrets.
- The resolved configuration is inspectable in CI without printing secrets.
- Tests prove a staging build cannot submit to production endpoints.
- Documentation lists the safe promotion process.

Start by locating the Expo release-profile configuration and the code that assembles API, Stellar, Sentry, and update-channel values. Trace how validation runs in CI, then add contract coverage for each profile, including the staging-to-production endpoint guard and secret-safe inspection. Done means invalid production configurations fail, resolved values are inspectable without secrets, and the safe promotion process is documented.

Written by the indexing model from the issue text.