[Frontend] Validate deployment configuration before hydration

## Problem
The frontend can start with a missing or mismatched API URL, Stellar network, contract address, or feature flag and fail only after a user connects a wallet.

## Scope
Create a typed runtime configuration validator that runs before client hydration and enforces network/address compatibility.

## Acceptance criteria
- Missing, placeholder, malformed, or cross-network values produce a clear non-sensitive startup error.
- Production configuration rejects testnet endpoints and development-only flags.
- Contract addresses are validated for network and checksum/format where applicable.
- The resolved config is exposed to diagnostics without secrets.
- CI tests every supported environment fixture and a negative matrix.

Start by locating the frontend client-hydration entry point and the existing configuration-loading path, then identify how supported environment fixtures are represented in CI. Done means a typed validator runs before hydration, reports non-sensitive errors for invalid or incompatible values, exposes a safe diagnostic config, and covers positive and negative environment cases in CI.

Written by the indexing model from the issue text.