Align activity-ui with datum-ui 2.9

[yahyafakhroji]

Problem
activity-ui carried its own copies of shadcn primitives next to datum-ui re-exports, imported Radix and cmdk directly, pinned a datum-ui peer (^0.8.0) that predates the picker family, had no UI checks in CI, and drew about two hundred hard-coded palette classes with hand-written dark variants. Its custom time range used native datetime-local inputs with no ordering check and a browser-drawn icon, and its infinite scroll never fired when the host page is the scroller. The example app could not server-render at all and 19 Playwright tests had rotted.

Solution
This branch now carries the whole alignment, merged in from #236, #237, and #238 after their reviews:

• Require datum-ui 2.9; delete the twelve local primitives the index already sources from datum-ui; rebuild filter chips on datum-ui popover and command; adapt combobox and multi-combobox; drop Radix and cmdk peers except tooltip; add a ui-tests CI job.
• TimeRangeDropdown wraps datum-ui's DateTimeRangePicker, keeping relative preset keys so URL state is unchanged; ActivityFeed observes the viewport when its container does not scroll.
• Every colour comes from datum-ui components or theme tokens; no palette classes or dark variants remain.
• All feed toolbar controls render at 36px with datum-ui's small Button and Input metrics, with a Playwright spec asserting one shared height.
  Public exports of ui/src/index.ts are byte-identical to before.

Closes datum-cloud/cloud-portal#1471, datum-cloud/cloud-portal#1472, datum-cloud/cloud-portal#1468. Design: docs/enhancements/ui-datum-ui-alignment.md

[kevwilliams]

The rewritten Combobox (ui/src/components/ui/combobox.tsx) drops the previous defaults clearable = false, showAllOption = true. SimpleQueryBuilder.tsx relies on those defaults for its Action/Resource/Namespace/User filters and never passes them explicitly, so all four filter dropdowns silently lose the "All" option and the clear-selection control. No e2e test covers this, so CI stays green despite the regression.

Fix: restore the previous defaults in Combobox, or pass showAllOption clearable explicitly in SimpleQueryBuilder.tsx, and add a Playwright spec asserting "All" still appears and clears the filter.

[yahyafakhroji]

@kevwilliams - Fixed in 5dd36ea. Combobox again defaults showAllOption to true and clearable to false, matching the version before the rewrite, so the four query-builder dropdowns get their "All" item back.

Two details beyond the report:

• The trigger label had regressed as well: with an empty value, datum-ui's Autocomplete showed the placeholder instead of "All". The "All" item now carries a sentinel value mapped to and from the empty string, so it resolves as the selected option and the trigger reads "All" when unfiltered, as before.
• clearable already defaulted to false before the rewrite, so no clear control was shown by default then either. /resource-history opts into it explicitly and is unaffected.