Refactor authentication: Switch from crypto to web2 and remove crypto-related code

[AdharBattulwar]

Summary
Switch the authentication mechanism from a crypto-based approach to standard web2 authentication in the dagent-api repository. Remove all crypto-related logic, dependencies, and UI elements in the auth forms and backend code that pertain to crypto wallets, signatures, or blockchain validation.

Tasks

• Migrate form logic to web2 authentication (e.g. username/password, OAuth, etc.)
• Remove references to wallet connection, Ethereum/Solana, signature verification, and related dependencies
• Refactor backend endpoints and validations away from crypto
• Update frontend auth forms to only show web2 login/register options
• Test web2 authentication flow and ensure crypto methods are fully removed

Additional Context

• This change will modernize and simplify authentication for broader accessibility
• If any part of the system relies on crypto-based auth for sensitive logic, document fallbacks or migration steps

Acceptance Criteria

• No user-facing or backend interface accepts/requires crypto wallet or digital signature
• All authed flows support only standard web2 authentication
• Documentation updated to reflect changes
• All removed code and dependencies are tracked and referenced in changelog

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Label suggestions: auth, web2, refactor, Feature