Add Freighter network validation with switch-network prompt

[Dannyswiss1]

The EVM reference site shows a "Switch to <network>" prompt when the connected wallet is on the wrong chain.
client/src/hooks/useWallet.ts fetches network via freighterApi.getNetwork() but nothing validates it against
an expected network (e.g. Testnet during development). Add an expected-network constant, a mismatch check, and a
Navbar prompt/button mirroring the reference site's UX, using Freighter's network-switch capability where
available.

Acceptance criteria

• Expected network is a single configurable constant (not hardcoded in multiple places)
• Navbar shows a distinct warning state when connected to the wrong network
• A test (per Component tests for the Toast provider/queue #9) covers the mismatch-warning rendering