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.

.env.example

@@ -65,10 +65,14 @@ TEXT_MODEL=gpt-oss:120b-cloud
 # VAPID_PUBLIC_KEY=
 # VAPID_PRIVATE_KEY=
 # VAPID_SUBJECT=mailto:you@example.com
-# Where subscriptions are stored (endpoint + keys only, no personal data).
+# Where subscriptions are stored (endpoint, keys, and the reminder text the
+# browser composed for that device — it may name one of its decks).
 # PUSH_DIR=.freezmo/push
-# The daily sender is a script you schedule yourself (cron/systemd timer):
+# The daily sender is a script you schedule yourself — cron, or the shipped
+# systemd timer (deploy/freezmo-reminders.timer):
 #   node scripts/send-reminders.mjs
+# A device's own reminder text wins; these generic lines are the fallback for
+# subscriptions that never synced one (same words the app uses without notes).
 # PUSH_REMINDER_TITLE=Time to review
 # PUSH_REMINDER_BODY=Your decks are waiting — a few due cards keep the streak alive.