Persist replacements across refresh (device-local, no account)

A reader who swaps a story away should keep that swap after a refresh; before,
the server re-served the original brief.
- localStorage now persists seen / dismissed / history (loadJSON/saveJSON).
- /api/brief accepts an exclude list; dismissed (replaced-away) ids are dropped
  and the highlights refill around them, so swaps stick and stay full.
- Replace records the swap to dismissed+seen and persists; the seen-set
  (persisted) keeps Replace from recycling across refreshes too.
- History panel survives refresh and gains 'Clear what I've seen (start fresh)'
  so it never feels suffocating. Saved history/favorites still come with sign-in.

Tests: brief exclude + refill (90 total).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/prefs.js

@@ -53,6 +53,24 @@ export function merge(userPrefs, moodFilter = {}) {
   return m;
 }
 
+// Generic device-local JSON storage (used for session memory: seen / dismissed
+// / history), so an account-less reader's choices survive a refresh.
+export function loadJSON(key, fallback) {
+  try {
+    const v = JSON.parse(localStorage.getItem(key));
+    return v == null ? fallback : v;
+  } catch {
+    return fallback;
+  }
+}
+export function saveJSON(key, value) {
+  try {
+    localStorage.setItem(key, JSON.stringify(value));
+  } catch {
+    /* private mode / quota — non-fatal */
+  }
+}
+
 // "" or "prefs=<encoded json>" for a query string.
 export function param(prefs) {
   const empty =