refactor(frontend): split manage tokens services in respective networks

[AntonioVentilii]

Motivation

To easy manageability and to prepare for more networks, we assign each manage-tokens service to each network folder.

PR Overview

This PR refactors the manage tokens services by splitting network-specific implementations into separate folders for Ethereum and ICP.

• Splits the previous token management service into network-specific versions.
• Introduces saveErc20UserTokens for Ethereum while replacing the ERC20 token service in the ICP network with saveIcrcCustomTokens.

Reviewed Changes

File	Description
src/frontend/src/eth/services/manage-tokens.services.ts	Adds a new function saveErc20UserTokens that wraps around the generic saveTokens service using the ERC20-specific saveUserTokens method.
src/frontend/src/icp/services/manage-tokens.services.ts	Removes the ERC20 token saving function and adds a new function saveIcrcCustomTokens to handle ICP custom tokens.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.