Fix class filter selection in WebUI

[briemla]

OP#10513

Problem

The class filter dialog of the WebUI (visualization_filters_form/container.py, open_classification_dialog) did not work:

• The checkboxes were created but never built, so the dialog showed only the "Apply" button.
• The on_value_change lambda bound the loop variable late, so every checkbox toggled the last class.
• The internal class list always started with all classes, regardless of the current selection (get_class_filter_selection()). With a partial selection, "Apply" also passed classes whose checkbox was not checked.

Solution

• The dialog is moved into its own class FilterClassificationForm (analogous to FilterDateRangeForm).
• Its state is derived from the current class filter selection. If there is no selection (None), all classes are selected, as in the customtkinter GUI. An empty selection ([]) now shows no class checked, which also matches the customtkinter GUI (previously it was treated like "all").
• Each checkbox is bound to its own class and evaluates the checkbox value instead of toggling.
• "Apply" passes exactly the checked classes, in the order of get_classes(), and closes the dialog.
• Buttons are built in _build_buttons, so a follow-up PR can add the reset button there (PR 6, webui/06-class-filter-reset, builds on this branch). The "Apply" label is still hard-coded; PR 6 moves it to the ResourceManager.

Tests

• New unit tests in tests/unit/OTAnalytics/plugin_ui/nicegui_gui/pages/visualization_filters_form/test_container.py (NiceGUI User fixture):
  • toggling a checkbox changes only its own class (unchecking and checking)
  • applying an unchanged partial selection passes exactly that selection
  • without a selection all classes are checked and applied
  • an empty selection checks no class
  • "Apply" closes the dialog
• The new tests fail on main (5 of 6) and pass with this change.
• pytest tests/unit/OTAnalytics/plugin_ui: 220 passed, 5 skipped
• pre-commit run --all-files: flake8, black, isort and mypy pass

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