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.

app/api/push/test/route.ts

@@ -61,13 +61,17 @@ export async function POST(request: Request) {
   }
 
   webpush.setVapidDetails(config.push.subject, config.push.publicKey, config.push.privateKey);
+  /* The test shows the reminder this device would actually get — the browser's
+     own text when it has one, the generic nudge otherwise — with a marker so a
+     test is never mistaken for the real thing. */
+  const message = target.message;
   try {
     await webpush.sendNotification(
       { endpoint: target.endpoint, keys: target.keys },
       JSON.stringify({
-        title: translate(locale, "server.pushReminderTitle"),
-        body: translate(locale, "server.pushReminderBody"),
-        url: "/",
+        title: `${translate(locale, "server.pushTestPrefix")} ${message?.title ?? translate(locale, "push.title")}`,
+        body: message?.body ?? translate(locale, "push.generic"),
+        url: message?.url ?? "/",
       }),
     );
   } catch (error) {