Extract hardcoded error strings into comprehensive public constants

Replace 60+ hardcoded error strings throughout transaction handlers with
public constants and templates for consistent error handling across the
Hotline protocol implementation.

Features:
- 33 authorization error constants (ErrMsgNotAllowed*)
- Account operation error constants (ErrMsgAccount*)
- File operation error templates with filename parameters (ErrMsgCannot*)
- Upload/download restriction templates (ErrMsgUpload*)
- Ban message constants (ErrMsgTemporaryBan, ErrMsgPermanentBan)
- General error constants (ErrMsgUserNotFound, ErrMsgCreateAlias)
- Chat/messaging templates (ErrMsgDoesNotAcceptTemplate)

Benefits:
- Single source of truth for all error messages
- Public API for other packages to import standard error constants
- Sprintf-style templates for dynamic content (filenames, usernames)
- Clear distinction between protocol errors (ErrMsg*) and golang errors
- Improved maintainability and consistency across transaction handlers

All error messages are now centralized, making future modifications
easier and ensuring consistent user experience across all operations.

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