fix(ui): address self-review findings (R13 S2c)

Three low-severity findings from the /code-review high self-pass:
- Plex duration: the "Any" chip no longer false-highlights when durationMin/Max
  hold a set-but-off-bucket value (stale/version-skew state) — it selects
  nothing then, matching the pre-sweep behaviour, instead of colliding on the
  empty-string sentinel.
- FilterChipGroup: count badge now guards on count > 0 (was `!= null`), matching
  the Tabs badge so a future count:0 can't render a stray "0" pill.
- Extract the shared CountBadge component; Tabs and FilterChipGroup now render
  the count-badge token from one source (was two near-identical inline spans).

gate green (tsc/eslint-0/vitest 136/knip/prettier). Duration self-E2E'd:
bucket select + Any toggle intact. Branch not merged.

frontend/src/components/Tabs.tsx

@@ -1,5 +1,6 @@
 import { type LucideIcon } from "lucide-react";
 import { useAccountPersistedState } from "../lib/storage";
+import CountBadge from "./CountBadge";
 
 // Reusable pill tab-rail for in-page sub-navigation (Configuration, Users & roles, the Settings
 // sections, the Stats overview/system split…). Horizontal by default; `orientation="vertical"` gives
@@ -55,15 +56,7 @@ export default function Tabs({
           >
             {Icon && <Icon className="h-4 w-4 shrink-0" />}
             {tb.label}
-            {tb.badge != null && tb.badge > 0 && (
-              <span
-                className={`text-2xs leading-none px-1.5 py-0.5 rounded-full ${
-                  on ? "bg-accent-fg/20" : "bg-accent/20 text-accent"
-                }`}
-              >
-                {tb.badge}
-              </span>
-            )}
+            {tb.badge != null && tb.badge > 0 && <CountBadge count={tb.badge} active={on} />}
           </button>
         );
       })}