refactor: unify datetime input handling and extend nicegui ui helpers

[onclefranck]

Summary

This PR brings together several improvements made on active_development before merging back into develop.

Included changes

• introduce a dedicated shared datetime_input component for split datetime handling
• remove transitional datetime legacy and align both pydantic and pandas with the unified component
• keep component_refs support coherent for split datetime fields
• extend NiceGUI runtime helpers with:
  • ui.builder(...)
  • ui.form_builder(...)
  • ui.table_builder(...)
• add workspace typing support so from nicegui import ui can expose those builder helpers in editor completion
• rename UI helper bindings to avoid collisions with NiceGUI's native ui.table(...)
• simplify packaging by removing the docs dependency extra
• align CI with the updated extras configuration

Validation

• full test suite passed locally: 264 passed

Notes

The NiceGUI helper integration is intentionally implemented as a package-owned convenience layer.
Runtime methods are attached by nicegui_builder, and editor completion for from nicegui import ui relies on local/workspace typing support rather than upstream NiceGUI changes.