Adds a reusable FilterSelect: a "Label: value ▾" pill that opens a listbox (a checkmark
row for single-select, a checkbox row for multi), on the AnchoredPopover platform. It
replaces the two feed-toolbar chip walls:
- Watch state (single-select, default All) — picking a row replaces + closes.
- Content types (multi-select, default Normal; requireOne keeps at least one on) — rows
toggle and the panel stays open.
The trigger summarises multi picks as "First +N". The component is source-agnostic, meant
for reuse anywhere a small known option set currently renders as a chip group.
FilterSelect takes a `testId` → the trigger gets it, each option gets `${testId}-${value}`,
so the feed/channel E2Es keep stable selectors. They now open the dropdown and assert the
option's aria-selected instead of the old inline aria-pressed chips (new expectContentType/
toggleContentType helpers).
peter
5851ddd329