Optimize project structure

[Junyi-99]

Refactor project structure by merging actor logic, renaming error handling, introducing a repository layer, reorganizing libraries, and renaming the webapp directory to improve clarity and maintainability.

This PR implements the specified project structure optimization plan, addressing several areas:

• Authentication Logic: Consolidated api/actor into accesscontrol to centralize user actor parsing and resolve potential circular dependencies with a UserExistenceChecker function type.
• Error Handling: Renamed the shared package to errors and updated all imports to use an apperrors alias, preventing conflicts with the standard library and improving semantic clarity.
• Data Access Layer: Introduced a dedicated repository layer to abstract data access logic from services, enhancing modularity and testability.
• Configuration Naming: Renamed the cfg library to config for clearer naming conventions.
• Webapp Directory: Renamed webapp/_webapp to webapp/extension to better reflect its purpose as a browser extension.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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