feat: implement balance fetch for Arweave, Litecoin, Monero, Decred

[oritwoen]

balance::fetch only handles Bitcoin and Ethereum - the remaining chains (Litecoin, Monero, Decred, Arweave) fall through to UnsupportedChain.

These variants have been in Chain since the early days but never got balance implementations. Now that Chain::from_str accepts all of them (#122), the gap is more visible.

Ref: #122 (comment)

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Balance fetching for Litecoin and Decred #117

🔗 Related PRs

#109 - refactor(chain): multichain prep [merged]

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