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.

deploy/freezmo-reminders.timer

@@ -0,0 +1,17 @@
+[Unit]
+# Freezmo — when the daily review reminder goes out.
+#
+# Evening by default: late enough that a day's study is already in the
+# schedule, early enough to still do something about it. `Persistent=true`
+# sends a missed run when the machine comes back (a laptop that was asleep),
+# and the random delay keeps every instance from firing on the same second.
+Description=Freezmo daily review reminder (timer)
+
+[Timer]
+OnCalendar=*-*-* 18:00:00
+Persistent=true
+RandomizedDelaySec=900
+Unit=freezmo-reminders.service
+
+[Install]
+WantedBy=timers.target