Improve Stellar network switching UI

[therealjhay]

Description: The app supports testnet and mainnet via NEXT_PUBLIC_STELLAR_NETWORK, but there's no UI to indicate which network the user is on, and no easy way to switch (it requires changing the env variable and restarting). For development, a network switcher is useful.

Requirements:

• Display the current network (Testnet/Mainnet) in the Topbar or footer
• Add a visual indicator: colored dot (yellow for testnet, green for mainnet)
• In development mode, add a network switcher toggle
• Show a warning banner when on testnet: "You are on Stellar Testnet — transactions use test tokens"
• Hide the switcher in production (or gate it behind an admin setting)

Suggested execution steps:

1. In components/layout/Topbar.tsx or Footer.tsx, add a network indicator
2. Read NEXT_PUBLIC_STELLAR_NETWORK from env
3. Display "Testnet" with a yellow dot or "Mainnet" with a green dot
4. Add a warning banner at the top of the app when on testnet
5. Optionally add a toggle for dev mode that reloads with a different network

Example commit message:

feat(stellar): add Stellar network indicator and testnet warning banner

Added network indicator (Testnet/Mainnet with colored dot) to the Topbar.
Shows a persistent warning banner "You are on Stellar Testnet" for
testnet environments. Helps users and developers identify the active
network at a glance.

[khanavi272-spec]

@khanavi272-spec has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue

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[adetomiwa21]

@adetomiwa21 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Kindly assign me this issue , thank you

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