Add Testnet indicator to Wallets and Confirmation screens

[Parsh]

This pull request introduces a visible Testnet indicator to core wallet screens, improving user clarity about the current Bitcoin network mode. When the app is in Testnet mode, the affected headers display a (Testnet) suffix; in Mainnet mode, no indicator is shown. The implementation is reactive to network changes, uses only display logic, and does not alter any data or navigation flows.

Header Indicator for Testnet Mode:

• Appends (Testnet) to the header titles on the Wallets tab, Keys tab, Receive Bitcoin screen, and Send Confirmation screen when bitcoinNetworkType is TESTNET, using a display-only suffix.
• Ensures indicator is non-interactive, introduces no layout shift, and does not affect any other UI elements or navigation.

Implementation Details:

• Uses useAppSelector to read bitcoinNetworkType in affected screens, ensuring the indicator updates reactively when the network changes without requiring a restart or navigation.
• Modifies header props and rendering in HomeScreenHeader, ReceiveScreen, and SendConfirmation to conditionally append the (Testnet) suffix.
• No new files, routes, components, or changes to Redux state shape, sagas, or data storage.

Testing and Risk Mitigation:

• Adds or updates render tests to verify the presence and absence of the indicator in all four locations for both Testnet and Mainnet modes.
• Risks (such as possible header truncation on very narrow screens) are documented and assessed as low impact.

Specification and Documentation:

• Provides detailed design, rationale, and requirements documentation, including affected files, migration plan, and non-goals for clear scope and future reference.

[cakesoft-vaibhav]

LGTM,
pls dev test this pr before merging