style/standardize network names

[Earnwithalee7890]

What changed? Why?
Standardized network name capitalization to Title Case across OnchainKit documentation:

• "Base mainnet" → "Base Mainnet"
• "Ethereum mainnet" → "Ethereum Mainnet"
• "Base testnet" → "Base Testnet"

Network names should be treated as proper nouns and capitalized consistently throughout the documentation.

Notes to reviewers

• Updated instances in wallet, paymaster, and configuration documentation.
• Only modified user-facing text (e.g., notes, comments, guide instructions).
• Code examples and variable names (e.g., mainnet imports from viem/chains) remain lowercase as they are technical identifiers.

How has it been tested?

• grep search to identify inconsistent network name capitalization.
• Manual review to ensure code variables and imports were not affected.
• Verified changes render correctly in local Mintlify preview.

[cb-heimdall]

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