feat(android): back the phone's contacts up to a group of one's own

A Contacts backup section on the Android Sync page sends a dated .vcf into
<folder>/<account>-contacts once a day when the address book changed, only
to a group the account is alone in, checked again at every run.

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

1 files changed, 18 insertions, 1 deletions

diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index ebcf152..9daf71b 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -408,7 +408,24 @@ of your groups, once a day.
 - **Where a photo was taken is removed** from the copy that is sent.
 - If the backup cannot go on — the node's disk is full, the folder no longer
   accepts files, you left the group — it stops, says why once in a
-  notification and in Settings, and tries again the next day.
+  notification and on the Android Sync page, and tries again the next day.
+
+### Backing up your phone's contacts
+
+**Contacts backup**, on the same **Android Sync** page, sends a copy of your
+phone's contacts to a group, once a day when they have changed.
+
+- **Only a group you are the only member of** can receive it: your address
+  book is not something to share. If you have none, create a group just for
+  yourself first. If somebody joins that group later, the backup stops and
+  tells you.
+- You choose any folder of that group you may add to; the copies go into a
+  folder named after you inside it, such as `Backups/alice-contacts`.
+- Each copy is a `.vcf` file with the date in its name
+  (`contacts-2026-10-10-0900.vcf`). Any phone can import it. Older copies are
+  kept, nothing is ever replaced.
+- 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.
 
 ### Search