Add the Sandbar mobile companion PWA

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/store.js

@@ -0,0 +1,41 @@
+// Settings shared by the page and the service worker. The service worker cannot read
+// localStorage, so the sign-in lives in IndexedDB; the page keeps a copy in localStorage
+// only for quick startup and falls back to IndexedDB.
+
+const DB = 'sandbar-mobile';
+
+function open() {
+  return new Promise((resolve, reject) => {
+    const r = indexedDB.open(DB, 1);
+    r.onupgradeneeded = () => r.result.createObjectStore('kv');
+    r.onsuccess = () => resolve(r.result);
+    r.onerror = () => reject(r.error);
+  });
+}
+
+export async function kvGet(key) {
+  const db = await open();
+  return new Promise((resolve, reject) => {
+    const r = db.transaction('kv').objectStore('kv').get(key);
+    r.onsuccess = () => resolve(r.result);
+    r.onerror = () => reject(r.error);
+  });
+}
+
+export async function kvSet(key, value) {
+  const db = await open();
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction('kv', 'readwrite');
+    if (value === undefined) tx.objectStore('kv').delete(key);
+    else tx.objectStore('kv').put(value, key);
+    tx.oncomplete = () => resolve();
+    tx.onerror = () => reject(tx.error);
+  });
+}
+
+// session: { server, token, keyId, user }
+export const getSession = () => kvGet('session');
+export const setSession = (s) => kvSet('session', s);
+// topics: the topics this device follows
+export async function getTopics() { return (await kvGet('topics')) || ['alerts']; }
+export const setTopics = (t) => kvSet('topics', t);