refactor: add BzzAddress to ChainConfig and remove postagecontract.LookupERC20Address

[martinconic]

Context

Follow-up from PR #5468 and the existing TODO at pkg/config/chain.go:14.

The BZZ token address is deterministic per chain, yet today it is resolved on-chain twice during node bootstrap:

• via the chequebook factory ERC20Address(ctx) → feeds erc20Service (used by /wallet and chequebook), and
• via postagecontract.LookupERC20Address at pkg/node/node.go:726 → feeds the postage stamp and staking contracts.

Proposal

• Add BzzAddress common.Address to ChainConfig (pkg/config/chain.go), populated for mainnet and testnet.
• Add a corresponding CLI/config override (e.g. --bzz-token-address) so custom/unknown chains can supply it.
• Replace the LookupERC20Address call site with chainCfg.BzzAddress and remove the function (and TestLookupERC20Address).
• Build erc20Service from the static address, decoupling /wallet from the chequebook factory entirely (simplifies the best-effort init added in fix(api): allow GET /wallet when swap or chequebook is disabled #5468).

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