fix(settings): sync across tabs; merge-on-write instead of clobbering

The settings atom was a load-time snapshot with no storage listener and
wrote the whole blob, so two tabs silently reverted each other. It now
re-reads on storage events and writes only the keys that changed
relative to the previous value. Unit-tested.

Fixes #42

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/settings.ts

@@ -473,19 +473,50 @@ export const getSettings = (): Settings => {
   }
 };
 
-export const setSettings = (settings: Settings) => {
+// Gitea #42 — merge-on-write. `next` is always this tab's whole-object view
+// (a shallow copy of what it last read, with one key changed — see
+// useSetSetting), which is stale the moment another tab has written since. Re-
+// reading the stored blob and reapplying only the keys that actually changed
+// relative to `previous` (this tab's own pre-update snapshot) means a change
+// this tab didn't make — e.g. a different setting toggled in another tab —
+// survives instead of being clobbered by this tab's stale snapshot of it.
+export const setSettings = (previous: Settings, next: Settings) => {
   try {
-    localStorage.setItem(STORAGE_KEY, JSON.stringify(settings));
+    const stored = getSettings();
+    const merged: Settings = { ...stored };
+    (Object.keys(next) as (keyof Settings)[]).forEach((key) => {
+      if (next[key] !== previous[key]) {
+        (merged as Record<keyof Settings, unknown>)[key] = next[key];
+      }
+    });
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(merged));
   } catch {
     /* quota */
   }
 };
 
 const baseSettings = atom<Settings>(getSettings());
+// Gitea #42 — settingsAtom used to be a one-shot snapshot with no cross-tab
+// sync at all (unlike atomWithLocalStorage.ts, which this mirrors). Without
+// this, a tab left open never sees settings changed in another tab until it
+// reloads, and (before the merge-on-write above) its next save would revert
+// them.
+baseSettings.onMount = (setAtom) => {
+  const handleStorageChange = (evt: StorageEvent) => {
+    if (evt.key !== STORAGE_KEY) return;
+    setAtom(getSettings());
+  };
+  window.addEventListener('storage', handleStorageChange);
+  return () => {
+    window.removeEventListener('storage', handleStorageChange);
+  };
+};
+
 export const settingsAtom = atom<Settings, [Settings], undefined>(
   (get) => get(baseSettings),
   (get, set, update) => {
+    const previous = get(baseSettings);
     set(baseSettings, update);
-    setSettings(update);
+    setSettings(previous, update);
   },
 );