swarm-ui: extract Card/MultiselectFilter/SplitView per component-first design

Mara: "result looks like the shape i am looking for, but the code does
not. you did not follow component first principle" - the card's
clickable/selectable mechanics, the card-view filter trigger+popover,
and the list+detail split layout were all one-off page-local JSX in
AgentsPage.tsx instead of docs/web-ui/design-guide.md's "Component-first
design" primitives. Three new ui/ components, each with a same-day
/components demo section per that doc's own rule:

- ui/card/Card.tsx - the role=button/keyboard-activation/selected
  mechanics AgentCard now wraps agent-specific content around, instead
  of owning them itself.
- ui/multiselect-filter/MultiselectFilter.tsx - the checkbox-list
  trigger+popover control. This was also a straight duplicate of
  Table's own inline popover content once AgentsPage's filter toolbar
  needed the identical thing; Table now renders the same
  MultiselectFilterOptions piece too (keeping its own th-anchored
  trigger and fixed+portal positioning, which are genuinely
  table-specific), not a second copy.
- ui/split-view/SplitView.tsx - the list+detail flex-wrap layout, no
  opinion on what's inside either pane.

AgentsPage.tsx's own CSS shrinks to just the agent-specific content
inside these primitives (card line/message layout, detail-panel field
grid, the name-search input) - the container/positioning rules moved
to each component's own colocated CSS.

No behavior change for any other Table caller (HivesPage,
IssueReportPage, the components demo's own Table samples) - the
popover's visual output is identical, just sourced from the shared
component instead of inline JSX.

Verified: typecheck/build clean, real screenshots of both AgentsPage
(pixel-identical to before) and the three new /components sections,
plus a live click confirming MultiselectFilter's popover opens
correctly on the demo page too.

frontend/packages/swarm-ui/src/ui/card/Card.css

@@ -0,0 +1,26 @@
+.ui-card {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35em;
+  padding: 0.75em 1em;
+  border: 1px solid var(--border);
+  border-radius: 0.5em;
+  background: var(--bg-elev);
+}
+/* Only an interactive `Card` (has `onClick`) gets pointer/hover
+   affordance — a plain static one shouldn't look clickable. */
+[role="button"].ui-card {
+  cursor: pointer;
+}
+[role="button"].ui-card:hover,
+[role="button"].ui-card:focus-visible {
+  border-color: var(--purple);
+  outline: none;
+}
+/* A solid border (not just the hover tint above, which needs to keep
+   meaning "hovering", not double as "selected") plus a faint fill so a
+   selected card still reads once the pointer moves away. */
+.ui-card-selected {
+  border-color: var(--purple);
+  background: color-mix(in srgb, var(--purple) 10%, var(--bg-elev));
+}