feat(android): back the calendars up as an iCalendar file

A Calendar section sends every calendar the person can edit, as a dated .ics,
into <folder>/<account>-calendar once a day when it changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 30 insertions, 7 deletions

diff --git a/docs/MESHBAY_DESIGN.md b/docs/MESHBAY_DESIGN.md
index 29b31a2..a30215d 100644
--- a/docs/MESHBAY_DESIGN.md
+++ b/docs/MESHBAY_DESIGN.md
@@ -3431,10 +3431,10 @@ foreground service (`BackupService`) and the WebView reported visible, like a
 cast (§11.3). Android 15 limits that type to six hours a day; the service stops
 when told and the run carries on at the next opening.
 
-### 9.13 Contacts and messages backup (Android)
+### 9.13 Contacts, calendar and messages backup (Android)
 
-The Android application sends a copy of the phone's **contacts** and its **text
-messages** to a folder of a group, on the photo backup's terms (§9.12): a client
+The Android application sends a copy of the phone's **contacts**, its
+**calendars** and its **text messages** to a folder of a group, on the photo backup's terms (§9.12): a client
 feature over the upload path, additive, once a day, the page doing the sending
 and the phone handing one file over by token (`/phonesync/<token>`,
 `phonesync/` in the Android package, `phone-sync.js` on the page). Each kind is
@@ -3452,6 +3452,18 @@ contact through `ContactsContract.Contacts.CONTENT_VCARD_URI`, the format any
 phone imports. It is sent only when its SHA-256 differs from the last copy the
 node took; a day with no change sends nothing.
 
+**Calendar.** `<folder>/<account>-calendar/calendar-2026-10-10-0900.ics`: one
+iCalendar file (RFC 5545, `Ics.kt`) of every calendar the person can write to
+(`CALENDAR_ACCESS_LEVEL` contributor or above); the ones they only subscribe
+to, public holidays or birthdays from contacts, are someone else's and come
+back by subscribing again. Timed events keep their Olson zone (`TZID`), so a
+weekly 9:00 stays 9:00 across summer time; recurrences keep their RRULE,
+RDATE and EXDATE, Android's `P3600S` durations become RFC's `PT3600S`, and a
+changed instance carries its series' UID and a RECURRENCE-ID. Reminders,
+organizer and attendees come along; each event names its calendar in
+`X-MESHBAY-CALENDAR`, which importers ignore. Like contacts it is sent when
+its SHA-256, taken without the DTSTAMP lines, differs from the last copy.
+
 **Messages.** `<folder>/<account>-messages/YYYY/sms-2026-10-10-0900.xml`: the
 SMS added since the last file the node took (the marker is the highest
 provider `_id` sent; ids only grow, so a message restored onto the phone later
@@ -3470,10 +3482,10 @@ answers null in `src/play`). The bridge offers `messageSync` only where the
 shell has a message source (`MESSAGES` in its prelude), so the Play build shows
 no Messages section rather than one that refuses.
 
-**Unlike photos**, both run on any network: a contacts file is small, and a
+**Unlike photos**, these run on any network: a contacts or calendar file is small, and a
 history of text messages is megabytes, not the gigabytes of a camera roll.
-`READ_CONTACTS` and `READ_SMS` are asked for when the person turns each on,
-never at start.
+`READ_CONTACTS`, `READ_CALENDAR` and `READ_SMS` are asked for when the person
+turns each on, never at start.
 
 ## 10. Filesystem portability
 
diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index e0c29da..1587160 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -385,7 +385,7 @@ that matters to you.
 ### Backing up your phone
 
 On the Android application, **Phone → Android Sync** in the side menu backs up
-your phone's photos, contacts and text messages to a group.
+your phone's photos, contacts, calendar and text messages to a group.
 
 Only your **personal profile** is backed up. If your phone has a work profile,
 nothing in it is sent, and MeshBay installed inside the work profile offers no
@@ -434,6 +434,17 @@ backup at all.
 - It runs on any connection, since a copy is small. **Back up now** sends one
   at once if your contacts changed since the last copy.
 
+#### Calendar
+
+- **Turn on** asks for access to your calendars, then sends a copy once a day
+  when they have changed.
+- It holds every calendar you can edit: your own, local ones and those of the
+  accounts on the phone. Calendars you only subscribe to (public holidays,
+  birthdays) are left out.
+- Each copy is an `.ics` file with the date in its name
+  (`calendar-2026-10-10-0900.ics`). Any calendar application can import it;
+  repeating events, their exceptions and reminders come along.
+
 #### Text messages
 
 - **Turn on** asks for access to your text messages. The first copy holds them