feat: add local-only completion and preference state

Completions and prefs live in localStorage and never reach the server —
there is no account and nothing to log into. Reads never throw, so a corrupt
value costs a preference rather than the whole screen.

Import merges and never removes: a completion present on either side stays
completed, because nothing else holds a copy to restore from.

The feed client refuses a schemaVersion it does not know rather than
guessing at unfamiliar fields.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/state/storage.ts

@@ -0,0 +1,40 @@
+/**
+ * localStorage access.
+ *
+ * Everything here stays on the device — there is no account, no session, and
+ * the server never learns what a user has completed. The `v1` segment in every
+ * key is the migration hook: read old versions forward, and never delete an old
+ * key until the migration has shipped and run, because a user who has not
+ * opened the app in six months still has their data under it.
+ */
+
+const NS = "gacha-tracker:v1";
+
+export const KEYS = {
+  completions: `${NS}:completions`,
+  prefs: `${NS}:prefs`,
+} as const;
+
+/**
+ * Reads never throw. A corrupt or foreign value falls back to the default
+ * rather than taking the app down — losing a preference is recoverable, a blank
+ * screen is not.
+ */
+export function readJson<T>(key: string, fallback: T): T {
+  try {
+    const raw = localStorage.getItem(key);
+    if (raw === null) return fallback;
+    return JSON.parse(raw) as T;
+  } catch {
+    return fallback;
+  }
+}
+
+export function writeJson(key: string, value: unknown): void {
+  try {
+    localStorage.setItem(key, JSON.stringify(value));
+  } catch {
+    // Quota exceeded or storage disabled (private mode). The UI keeps working
+    // from in-memory state; only persistence is lost.
+  }
+}