Fix issue #306 in nextjs-anchor template

[cxalem]

This PR addresses the issues Jonas reported in #306 for kit/nextjs-anchor.

I updated the template UX and flows with cluster switching, wallet dropdown connection and error handling, wallet balance and airdrop, explorer links, and clearer transaction and error messaging in the vault interactions. I also fixed the template docs and setup text so it matches the real workflow.

I replaced framework-kit usage with a self-contained app/lib layer built on @solana/kit and wallet-standard for wallet connection, transaction sending, cluster RPC, and balance hooks.

If this direction looks good, I can replicate the same design identity and lib structure across the other kit templates.

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P.D.

This implementation increases the boilerplate, since we removed a library to add custom code, and we'll need to do this in all the templates. The right direction is to make this a library or use an existing one!

[cxalem]

I've addressed all the review comments:

• Vault PDA derivation, instructions, and error codes now come from the Codama-generated IDL client
• Balance updates use WebSocket subscriptions (with SWR polling as fallback)
• Magic strings replaced with constants
• Broken documentation links fixed

We decided to continue with the current approach, adding a self-contained app/lib layer on top of @solana/kit and wallet-standard. This does add some boilerplate, but it keeps things working and explicit until the Kit plugins are stable enough to integrate into the templates. This pattern is designed to be replicated across all the kit templates.

[github-actions]

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