Extract a shared popover controller for dropdown, listbox, and combobox

[ukutaht]

Dropdown, listbox, and combobox duplicated popup positioning, dismissal, and focus handling. Extract a shared internal popover controller so all three use consistent open state, ARIA/inert updates, focus return on dismissal, and Floating UI positioning. Selection and option navigation remain in the component hooks.

• Tab and Shift+Tab dismiss the popup and continue focus navigation. Combobox dismissal discards unfinished search while preserving committed selections.
• Reuse delegated listeners across LiveView updates and batch combobox selection updates.
• Replace match_trigger_width with --reference-width, available to all three components and measured from the configured reference. Use min-w-[var(--reference-width)] for the previous minimum-width behavior or w-[var(--reference-width)] for an exact match. Listbox width matching now requires CSS; examples and documentation are updated.
• Consolidate shared lifecycle tests and add coverage for CSS sizing, resizing, and custom references.