build the app, and ship it on Linux and Windows as well as macOS

The client itself: the sync engine and mirror, the Gmail and IMAP providers,
the screener, the three boxes and two piles, compose and send, the sanitiser
and tracker stripping, contacts, clips, backup, notifications and the guide.

The pipeline that ships it. Three runners now build in parallel and the
publish gate wants all four platform keys in latest.json before a release
leaves draft. Linux gets two more packages Tauri does not build: a flatpak
repackaged from the deb against GNOME 48, and a Nix package that relinks the
deb against nixpkgs so it runs as a native Wayland client rather than through
Xwayland. CI runs the Rust suite on all three desktops rather than on Linux
alone, and rebuilds the flatpak manifest on every push to main.

Two things that were only ever exercised on macOS and were wrong everywhere
else. The menu bar was built by adjusting the submenus macOS is given, so on
a platform whose default menu has no File or View the new File menu landed
after Edit, Check for Updates landed nowhere, and the three places and the
reading pane had no menu at all. The deb declared libwebkit2gtk-4.1-0 and
libgtk-3-0 twice, because tauri.conf.json named the dependencies Tauri
already emits.

The updater key exists now, so releases can sign their artifacts.

src-tauri/fixtures/nested-multipart.eml

@@ -0,0 +1,57 @@
+Return-Path: <[email protected]>
+MIME-Version: 1.0
+Date: Wed, 2 Sep 2026 16:04:51 -0500
+Message-ID: <[email protected]>
+Subject: Bill payment pending
+From: City Power <[email protected]>
+To: Priyanshu Jain <[email protected]>
+Auto-Submitted: auto-generated
+Content-Type: multipart/mixed; boundary="outer-116400046977232"
+
+--outer-116400046977232
+Content-Type: multipart/alternative; boundary="inner-116400046977232"
+
+--inner-116400046977232
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 7bit
+
+Dear Customer,
+
+Your one-time electronic payment of 98.26 for your City Power bill has
+been received and is pending. Your payment will be posted within two
+business days.
+
+Confirmation number 116400046977232
+
+Please keep this message for your records. If you did not make this
+payment, call us on the number printed on your statement.
+
+Thank you,
+City Power Customer Care
+
+--inner-116400046977232
+Content-Type: text/html; charset=UTF-8
+Content-Transfer-Encoding: quoted-printable
+
+<html><body style=3D"font-family:Arial,sans-serif">
+<p>Dear Customer,</p>
+<p>Your one-time electronic payment of <b>98.26</b> for your City Power =
+bill has been received and is pending. Your payment will be posted with=
+in two business days.</p>
+<p>Confirmation number 116400046977232</p>
+<p>Thank you,<br>City Power Customer Care</p>
+</body></html>
+
+--inner-116400046977232--
+
+--outer-116400046977232
+Content-Type: application/pdf; name="Statement-August.pdf"
+Content-Transfer-Encoding: base64
+Content-Disposition: attachment; filename="Statement-August.pdf"
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+
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