ui: drop the filter's bottom margin — the list's top padding is the seam

The filter's mb-2 stacked with the session list's built-in 8px top
padding, doubling that seam to 16px while the others read 8. The
column now runs 12 / 8 / 8 / 8 — the list's own padding carries the
last seam.

components/sidebar.tsx

@@ -757,7 +757,7 @@ function SessionsSidebar() {
 				    Practice / All — the real modes first, "All" as the escape
 				    hatch on the right, mirroring the create-buttons above.
 				    Persisted client-side. */}
-				<div className="mx-3 mb-2 flex items-center gap-0.5 rounded-md border border-border/60 bg-background p-0.5" data-testid="session-kind-filter">
+				<div className="mx-3 flex items-center gap-0.5 rounded-md border border-border/60 bg-background p-0.5" data-testid="session-kind-filter">
 					{(["teach", "practice", "all"] as const).map((f) => (
 						<button
 							key={f}