Unit tests for QML Pickers and Selectors #360

Part of #356

Add unit tests in tests/qml/ for active pickers and selectors:

• DateRangeSelector.qml (date bounds and range pick)
• DaySelector.qml (day selection signals)
• DateRangeHeaderFilter.qml (filter toggle and active range text)
• FilterableHeaderContents.qml (filter state integration)
• SelectionButton.qml (selection state and click)
• StatusSelector.qml (status changes and badge color)
• InlineOptionSelector.qml (radio-style option selection)
• MultiAssigneeSelector.qml (multi-select IDs array output)
• WorkItemSelector.qml (project and task selection logic)
• ProjectStageSelector.qml (stage list per project)
• StageFilterMenu.qml (stage filter menu toggling)
• AssigneeFilterMenu.qml (assignee filter menu toggling)

Note: Unused selectors (AccountSelector, ProjectSelector, UserSelector, TreeSelector, DialogComboSelector, OptionSelectorPopover, QuickDateSelector) are tracked for removal in #367.