feat: shared backlog filter popover + sprint header polish (v1.3.3) (#184)

- Move sprint switcher into sprint header, centered between title and actions
- Extract BacklogFilterPopover as shared component used by sprint and product backlog
- Add sort options (code/priority/status) with single-pill asc/desc toggle
- Default sprint backlog status filter to OPEN, remove "alleen niet klaar" button
- Persist collapsed state and filter popover open in localStorage
- Fix hydration flicker: defer localStorage read to useEffect with prefsLoaded gate for writes
- Increase sprint switcher text size for readability

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/use-local-storage-pref.ts

@@ -0,0 +1,22 @@
+/**
+ * SSR-safe synchronous read of a localStorage value with a typed parser.
+ *
+ * Use inside `useState(() => readLocalStoragePref(...))` so the first render
+ * already has the persisted value — no useEffect-driven re-render flicker.
+ *
+ * On the server `window` is undefined → returns `fallback`. On the client the
+ * raw value is parsed; if the parser returns `null` the fallback is used.
+ * Hydration mismatches between server-rendered HTML (default) and the
+ * client-rendered tree (persisted) are accepted: React adapts the DOM in the
+ * same hydration pass without a visible flicker for matching values.
+ */
+export function readLocalStoragePref<T>(
+  key: string,
+  parse: (raw: string) => T | null,
+  fallback: T,
+): T {
+  if (typeof window === 'undefined') return fallback
+  const raw = window.localStorage.getItem(key)
+  if (raw === null) return fallback
+  return parse(raw) ?? fallback
+}