style: match Tasks filter button stroke to sibling toolbar buttons

Add buttonVariant="outline" to the IssueFiltersPopover usage in the Tasks
toolbar so the funnel button matches the neighboring outline icon buttons
(column picker, sort), consistent with the Inbox call sites. Opt-in at the
call site only; the popover default stays ghost.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssuesList.tsx

@@ -1481,6 +1481,7 @@ export function IssuesList({
             currentUserId={currentUserId}
             enableRoutineVisibilityFilter={enableRoutineVisibilityFilter}
             iconOnly
+            buttonVariant="outline"
             workspaces={isolatedWorkspacesEnabled ? workspaceOptions : undefined}
           />