fix(calendars): hide-only visibility reconcile, and keep it working read-only

Code review of the one-visibility-model fix (#75) found the reconciliation
reaching further than it should and the read-only case falling through it.

The migration switched calendars *on* to keep the upgrade invisible, but
"not disabled in Calendula" is the default for every calendar, including ones
the user deliberately hid in Google Calendar, Etar or DAVx5 — those would
reappear there and start firing reminders from a switch the user never touched.
Its sync_events guard didn't hold either: an ACCOUNT_TYPE_LOCAL calendar another
app created can sit at sync_events=0 while holding real device-local events. The
reconcile now only hides, and a one-time notice explains that visibility follows
the device and where to change it, instead of quietly rewriting other apps'
state.

Only READ_CALENDAR gates the app, so a read-only install could not write the
flag at all: every calendar it had switched off came back with its events and
its reminders, and the switch couldn't undo it. Those switch-offs are kept
app-side now (the retired disabled-set key, re-read under a new name), folded
into the visibility every consumer reads, and drained into the provider entry by
entry once WRITE_CALENDAR arrives — which also makes a part-applied run resumable
without re-applying a switch the user has since flipped by hand.

Also: restore the ReminderNotifier.post gate, the one path a snooze re-shown
from our own alarm passes; move the whole reconcile inside its try/catch, so a
damaged preferences file can't crash the process at launch; share one Calendars
query per provider tick across the flows that need it; and give the reworded
Settings hint new keys, so five locales stop rendering the retired app-only
wording.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/main/res/values-pl/strings.xml

@@ -396,8 +396,6 @@
     <string name="calendars_local_header">Twoje kalendarze</string>
     <string name="calendars_local_empty">Brak lokalnych kalendarzy. Utwórz kalendarz, aby zapisywać wydarzenia tylko na tym urządzeniu.</string>
     <string name="calendars_add">Dodaj kalendarz</string>
-    <string name="calendars_disable_hint">Wyłącz kalendarz, aby ukryć go w aplikacji — wraz z jego wydarzeniami, filtrami i selektorami. Nic nie zostanie usunięte, a w każdej chwili możesz go tutaj ponownie włączyć.</string>
-    <string name="calendars_show_in_app_a11y">Pokaż „%1$s” w aplikacji</string>
     <string name="calendars_synced_header">Synchronizowane kalendarze</string>
     <string name="calendars_synced_hint">Pochodzą z kont na Twoim urządzeniu. Możesz je tworzyć i edytować w powiązanych z nimi aplikacjach.</string>
     <string name="calendars_manage_in_app">Zarządzaj w aplikacji</string>