Put address books in the left pane, other people's included

Address book sharing was withdrawn a few hours ago on a report that it
behaved like mail folder sharing. That was wrong -- it works -- and it is
back, built the way Files is rather than the way it was.

Three things it inherits from Files. Shared books are listed in the app's
own left pane instead of behind an account switch in the profile menu.
The reader's books and other people's sit under separate headings, since
a book belonging to somebody else behaves differently and a single merged
list would be quiet about whose contacts you are reading. And opening
Contacts re-reads the session, so a book shared while the tab was open
turns up without signing out and in again.

The books pane the view kept to itself is gone, and with it the last
module that ignored the sidebar it was given.

The one thing Files does not need: shared contacts have to answer when
somebody types a name into a To field, so they are loaded up front rather
than when a book is opened, and they are offered by `suggest` and found
by `lookupByEmail` alongside the reader's own. Their own cards win a tie,
since a card someone wrote themselves should beat a colleague's copy of
the same person. That is the difference between a shared book you can
look at and one you can use.

Cards from a shared account are held apart from the reader's rather than
merged in, and keyed by account as well as id. Ids are only unique within
an account -- two accounts each having a book `ab1` is ordinary -- and a
flat map would have had one silently replace the other.

The mock grew an address book in its shared account, with contacts in it,
because none of this could be exercised otherwise.

KNOWN-ISSUES records the withdrawal as the mistake it was rather than
leaving it in the history looking like a finding. Mail folder sharing
stays withdrawn: that one really is broken.

KNOWN-ISSUES.md

@@ -23,7 +23,7 @@ works the same way — and dropped where 0.15 was the whole subject. Support for
 
 - **Delete all spam destroys, and does not pass through Deleted Items** — this is the point of the feature and the thing worth checking on a real server, since a folder that empties into another folder has solved nothing. `Email/set destroy`, walked a page at a time so it survives `maxObjectsInSet` the way emptying Deleted Items already had to. **Confirmed live on 0.16.19 (2026-08-26)**: Junk Mail emptied and Deleted Items stayed empty afterwards. There is no undo, which is why all three entry points share one dialog that says so. Only Deleted Items and Junk Mail can be emptied this way, enforced in the store rather than only hidden in the menus.
 - **Sharing a mail folder is accepted and does nothing.** `Mailbox/set` with a `shareWith` map is applied, `Mailbox/get` reads it back, and the folder never appears for the account it was shared with — **confirmed live on 0.16.19 (2026-08-27)** with a folder shared read-only to another account on the same server, which never saw it. Stalwart's own sharing documentation lists calendars, address books and file storage; mail folders are not among them. Nothing reports a failure at any point, which is the whole problem: the share is stored, so a client that trusts what it reads back shows it as live for ever. The entry point is withdrawn. A folder that is *already* shared still offers **Stop sharing**, because a share nobody can see is exactly the one you want to be able to clear, and there is no other way to. File sharing is unaffected and works end to end.
-- **Address book sharing is withdrawn without being disproved.** It was taken out alongside mail folders on 2026-08-27, on a report that it behaved the same way, and that report has not been reproduced: there was no shared address book left on the account by the time anyone looked. Stalwart documents address books as shareable, so the expectation is that this one *does* work and the entry point should come back — it is out because offering a share nobody can verify was worse than the gap. Testing it needs two accounts and someone to confirm the book arrives. **Stop sharing** remains for a book already shared.
+- **Address book sharing works, and was briefly withdrawn by mistake.** It was taken out alongside mail folders on 2026-08-27 on a report that it behaved the same way; the report was mistaken and the feature was put back the same day. Nothing was ever shown to be wrong with it, and Stalwart documents address books as shareable. Recorded because the withdrawal is in the history and would otherwise read as a finding. Shared books now appear in the Contacts pane under "Shared with me" rather than behind an account switch, and their contacts are offered when addressing a message.
 - **Read receipts are built here, not by the server** — JMAP has an extension for them, [RFC 9007](https://www.rfc-editor.org/rfc/rfc9007.html)'s `MDN/send`, and Stalwart does not implement it: `urn:ietf:params:jmap:mdn` is not among its capabilities. So ihasmail assembles the `multipart/report` itself and sends it the long way round — raw MIME uploaded as a blob, `Email/import`, then `EmailSubmission` — which is also why the receipt lands in Sent, where it honestly belongs. Non-ASCII parts are base64 rather than `8bit`, so nothing depends on 8BITMIME surviving every hop. There is deliberately no "always send" setting: a receipt confirms to whoever asked that the address is live and when it was read, to an address of the sender's choosing, so each one is a decision. Verified against the mock end to end (upload, import, submit, `$mdnsent`), and **confirmed live on 0.16.19 (2026-08-26)**: a receipt asked for by a real sender was assembled, uploaded, imported and submitted, landed in Sent, and set `$mdnsent` so a second look does not offer to send another.
 - **Where 0.16 advertises `urn:stalwart:jmap`** — not where a JMAP client would look, and this now decides whether a sign-in is allowed at all. Stalwart builds the session-level `capabilities` from a fixed list (`Session::new`, plus WebSocket) that has never contained this capability, in any 0.16.x from 0.16.0 to 0.16.19. It hands it out per-account instead, so it appears in `primaryAccounts` and in each account's `accountCapabilities`. ihasmail tested for it in `capabilities` alone, which made every real 0.16 server read as older than 0.16 — and that one check drove three things: self-service credentials fell back to `POST /api/account/auth`, which 0.16 removed, so password changes, 2FA and app passwords all failed with "this mail server does not offer self-service credential management"; About reported the wrong generation; and Files took the older code path. It now looks in all three places, and is covered by tests on each. Worth restating plainly, because the stakes went up when 0.15 support was dropped: there is no longer a fallback path for this check to be wrong *into*. Getting it wrong now refuses every sign-in against a perfectly good server — a loud failure rather than a quiet misrouting, which is the trade the removal was making.
 - **HTML signatures** — Stalwart caps a signature at 2047 **bytes** (`value.len() < 2048` on a Rust string, so UTF-8 bytes, not characters). ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker; other clients see a text fallback. Confirmed live on 0.15.5 (2026-08-24): oversized, non-ASCII and inline-image signatures all save, and a test message arrived intact at Gmail with the logo inline.