Push reminders: personal to the notes, generic without them

The daily reminder said one generic line to everyone, because the server
never sees a deck. Now the notification text is written in the browser
(lib/reminder.ts) and travels with the subscription:

- Personal: "3 cards are due in Sky deck", in the app's language, tapping
  through to /review. Without notes (or with nothing due) it is the plain
  nudge, translated in both languages — push.title / push.due.* / push.generic.
- POST /api/push accepts an optional message {title, body, url}; url must be
  a path on this app, so a stolen subscription cannot make the notification
  link elsewhere. The store keeps endpoint, keys, the message and createdAt
  (a refresh no longer resets the timestamp).
- components/PushReminderSync.tsx refreshes the stored text when the app
  opens and whenever the tab becomes visible (throttled to twice a day), so a
  count is only ever sent while it is true.
- scripts/send-reminders.mjs sends each device its own text and falls back to
  PUSH_REMINDER_TITLE/_BODY for a subscription that has none
  (scripts/reminder-payload.mjs, unit-checked).
- /api/push/test previews the real reminder with a "Test:" prefix.
- deploy/freezmo-reminders.{service,timer}: the daily send as a systemd user
  timer (18:00 local, Persistent, randomised), documented in the README.
- Settings copy, privacy page (en+cs) and .env.example say exactly what the
  server now holds.
- phase2-test grew the whole path with a faked browser subscription: personal
  text in en and cs, the generic fallback in both, the URL guard, and the
  sender's preference order (81 checks).

Version 0.8.2, SW v104.

lib/reminder.ts

@@ -0,0 +1,60 @@
+"use client";
+
+/**
+ * The daily reminder's text, composed **in the browser**.
+ *
+ * The server never sees the notes — it only knows which push endpoints exist —
+ * so the personal part of the nudge has to be written here and stored with the
+ * subscription (see lib/push-store.ts). Personal means: how many of the user's
+ * cards are due and which deck they belong to; the notification then opens the
+ * review screen. When there is nothing to name — no decks yet, or nothing due —
+ * the message is the plain nudge in the user's own language (English or Czech,
+ * like every other string in the app).
+ *
+ * Always resolves; a failure to read the local stores only costs the personal
+ * words, never the reminder.
+ */
+
+import type { SrsRecord } from "./history";
+import { ensureLearningHydrated, learningStore } from "./history";
+import { plural, t } from "./i18n";
+import { dueItems } from "./learning";
+import { deckStore } from "./storage";
+
+export type ReminderMessage = { title: string; body: string; url: string };
+
+/** The generic nudge, used whenever the notes cannot carry the message. */
+export function genericReminder(): ReminderMessage {
+  return { title: t("push.title"), body: t("push.generic"), url: "/" };
+}
+
+/** Which deck has the most due cards (ties keep the first, i.e. the oldest). */
+function busiestDeck(schedules: SrsRecord[]): { title: string; count: number } | null {
+  const byDeck = new Map<string, { title: string; count: number }>();
+  for (const item of dueItems(schedules)) {
+    const entry = byDeck.get(item.deck.id) ?? { title: item.deck.title, count: 0 };
+    entry.count += 1;
+    byDeck.set(item.deck.id, entry);
+  }
+  let top: { title: string; count: number } | null = null;
+  for (const entry of byDeck.values()) {
+    if (!top || entry.count > top.count) top = entry;
+  }
+  return top;
+}
+
+export async function buildReminderMessage(): Promise<ReminderMessage> {
+  try {
+    await ensureLearningHydrated();
+    const top = busiestDeck(learningStore.getSrsSnapshot());
+    if (!top || deckStore.getSnapshot().length === 0) return genericReminder();
+    return {
+      title: t("push.title"),
+      body: plural(top.count, "push.due", { deck: top.title }),
+      /* The review screen holds the due queue the nudge talks about. */
+      url: "/review",
+    };
+  } catch {
+    return genericReminder();
+  }
+}