No connection status indicator for the Stacks network in the header

[Mosas2000]

Description

The header shows the connected wallet address but gives no indication of which Stacks network the application is connected to (mainnet, testnet, devnet) or whether the Stacks API is reachable. Users should always know what network they are interacting with to avoid sending real STX during testing.

File

frontend/src/components/Header.jsx

Suggested Fix

Add a network badge in the header that displays the current network (Mainnet, Testnet, Devnet) with appropriate color coding (green for mainnet, yellow for testnet, blue for devnet). Include a small indicator showing whether the Stacks API is reachable. On testnet, show a prominent warning banner.