[bugfix] Fix alert label display and enable custom label input

[P-Peaceful]

What's changed?

This PR fixes two UI issues with alert labels reported in #3901:

1. Auto-added labels from severity selection are now properly displayed in the UI
2. Users can now add custom labels via free text input

Changes

Backend

• Refactor: Moved LabelDao and LabelService from hertzbeat-manager to hertzbeat-base module to enable cross-module usage
• Feature: Added saveNewCustomLabel() method in AlertDefineServiceImpl to automatically persist custom labels when creating/updating alert definitions

Frontend

• Feature: Enabled custom label input by setting labelIsCustom: true in alert-setting component
• UI: Users can now type custom key-value pairs in addition to selecting from existing labels

Screenshot

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.