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

CHANGELOG.md

@@ -7,6 +7,28 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Settings → Calendars now says what is different about a calendar instead of
+  leaving you to guess. Ones you can only view — a subscribed calendar, a
+  calendar shared with you read-only — are marked **Read-only** ([#76]).
+- Calendars your device isn't syncing are marked **Not synced**, moved to the
+  bottom of their account and left without a switch. None of their events are on
+  the device, so the switch they used to have could not have shown you anything
+  — the calendar simply looked broken. They are no longer offered when you pick
+  a calendar for a new or an imported event either: an event saved there would
+  never reach the account. Whether an account syncs a calendar stays that
+  account's own app's decision ([#78]).
+- The birthday and anniversary calendars Calendula fills from your contacts are
+  marked **Filled from your contacts**, which is why they can't be picked for a
+  new event: anything you put there would be removed again on the next sync.
+  Deleting one is held back while special dates are switched on — Calendula
+  would simply create it again — and the calendar's editor says so; turn the
+  feature off under Settings → Special dates and the delete works as usual
+  ([#76]).
+- The calendar picker in the event form and in the .ics import screen now ends
+  with a **"Missing a calendar?"** row that opens Settings → Calendars, where
+  those marks then explain why a calendar isn't offered ([#76]).
+
 ### Fixed
 - Reminders now arrive for every calendar you have switched on. A calendar that
   was hidden at system level — switched off in another calendar app, or never
@@ -1135,3 +1157,5 @@ automatically, with zero telemetry and no internet permission.
 [#44]: https://codeberg.org/jlmakiola/calendula/issues/44
 [#70]: https://codeberg.org/jlmakiola/calendula/issues/70
 [#75]: https://codeberg.org/jlmakiola/calendula/issues/75
+[#76]: https://codeberg.org/jlmakiola/calendula/issues/76
+[#78]: https://codeberg.org/jlmakiola/calendula/issues/78