shared/dropdown: optional portal mode escapes a scroll-container clip

A `WantedMenu` badge dropdown on the agents table clips against
`.ui-table-scroll`s overflow the moment its row is the last (or
near-last) one — the popover extends past the table content the
scroll container bounds itself to. `Table.tsx` already solved the
identical clip for its own column-filter popover with a
position:fixed + portal computed from the anchor rect; `Dropdown` now
takes an optional `portal` prop that opts a caller into that same
recipe instead of a second hand-rolled copy of it. Off by default —
every other current caller (StatusChips x2, the components-page demo)
keeps its existing non-portal behavior unchanged.

frontend/packages/swarm-ui/src/pages/AgentsPage.tsx

@@ -213,6 +213,7 @@ function WantedMenu({
       />
       <Dropdown
         open={open}
+        portal
         options={options}
         activeValue={row.wanted ?? undefined}
         label={`declare ${row.name}`}