feat(calendars): explain a calendar's state, and why one is missing from the picker (#76, #78) (!101)

Follow-up to #97, on the same release line. After the visibility fix a calendar can be absent from the pickers for three different reasons the app knows and never said; this closes that gap.

**Settings → Calendars names the state** (#76, #78): read-only calendars are marked `Read-only`; ones whose account isn't syncing events to this device are marked `Not synced`, sorted to the bottom of their account, dimmed, and left **without a switch** — visibility can't reveal events that aren't on the device, so the control did nothing.

**Both pickers end in a "Missing a calendar?" row** (#76) opening the calendar manager, where those labels then say which reason applies. The manager overlay moved to the end of the host's overlay stack so it covers every surface that can open it (Settings, both event forms, the import picker).

Review notes:

- **Supporting text, not chips.** A row can carry several states at once next to a live switch; M3 supporting text composes there, static badges don't (and a non-interactive chip reads as a broken button).
- **`sync_events = 0` counts as "not synced" for account-backed calendars only.** Nothing syncs a device-local calendar by definition, and another app's local calendar can hold real events at 0 — the unsoundness that made the first #75 migration guard wrong. Covered by a test.
- **Excluded calendars stay out of the pickers** rather than being listed unpickable — a handful of read-only subscriptions would crowd out the ones you can actually choose.

541 JVM tests green (6 new), lint clean, check_translations clean. **On-device review owed**; the three new string keys need the Weblate backfill.

Reviewed-on: #101

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

@@ -477,6 +477,13 @@
     <string name="calendars_visibility_a11y">Show \"%1$s\"</string>
     <string name="calendars_visibility_notice_title">Some calendars are switched off</string>
     <string name="calendars_visibility_notice_message">Calendula now shows the calendars that are switched on for this device, so what you see and what reminds you can no longer disagree. Some of yours are currently off — they were switched off here or in another calendar app. Turn any of them back on in Settings → Calendars.</string>
+    <!-- Footer row under the event-form and .ics import calendar pickers. -->
+    <string name="calendar_picker_missing_title">Missing a calendar?</string>
+    <string name="calendar_picker_missing_summary">It may be switched off, read-only, or filled from your contacts — manage your calendars here.</string>
+    <string name="calendars_state_read_only">Read-only</string>
+    <string name="calendars_state_not_synced">Not synced to this device</string>
+    <string name="calendars_state_managed">Filled from your contacts</string>
+    <string name="calendars_managed_delete_locked">This calendar is filled from your contacts, so Calendula would create it again on the next sync. Turn special dates off under Settings → Special dates to delete it.</string>
     <string name="calendars_synced_header">Synced calendars</string>
     <string name="calendars_synced_hint">These come from accounts on your device. Create and edit them in their own app.</string>
     <string name="calendars_manage_in_app">Manage in app</string>