a11y: make search facet pickers proper comboboxes

[emmpaul]

Problem

The search page's author/channel facet pickers (resources/js/pages/channels/Search.vue, introduced in #395) are implemented as a DropdownMenu with a filter <Input> inside it. This is functional and keyboard-operable (Tab between the filter and the option buttons, Enter to select, Escape to close, type-to-filter) and the page passes the axe audit in both themes with the pickers closed — but it is not a proper combobox pattern:

• The filter input uses @keydown.stop to keep the menu's type-ahead from hijacking typing, which also blocks arrow-key roving to the options.
• The option rows are plain <Button>s inside DropdownMenuContent rather than registered menu/listbox items, so there is no roving aria-activedescendant navigation.
• A text input inside a menu role is an ARIA anti-pattern; the open state is not axe-audited.

Surfaced by a local CodeRabbit review on the #391 search-redesign PR; deferred to keep that (already large) PR scoped.

Acceptance criteria

• The author and channel facet pickers expose proper combobox semantics (a role="combobox" input with aria-expanded/aria-controls, a listbox of options, roving aria-activedescendant or focus), reusing/introducing a shared primitive (reka-ui Combobox/Listbox or the existing command component) rather than a DropdownMenu.
• Full keyboard flow: open, type to filter, arrow to navigate, Enter to select, Escape to dismiss.
• Filtering + single-select behavior preserved; data-test hooks preserved so the existing browser tests keep passing.
• An axe audit runs against the open picker in a Pest browser test, in light and dark themes.