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/golden/encoded-words-split-utf8.txt

@@ -0,0 +1,35 @@
+render-version: 2
+message-id: [email protected]
+in-reply-to: -
+references-first: -
+from: Airbnb <[email protected]>
+to: <[email protected]>
+cc: -
+bcc: -
+reply-to: Inês <[email protected]>
+date-ms: 1788406400000
+subject: Your reservation in Lisbon is confirmed, Inês is expecting you
+is-html: false
+surface: theme
+snippet: Check-in Friday 12 September after 15:00. Your host Inês will send the door code the evening before. Rua da Bica de Duarte Belo 42, Lisboa. The tram stops at the top of the street and the walk down…
+list-id: -
+auto-submitted: -
+precedence: -
+unsubscribe: -
+blocked-images: 0
+
+--- html ---
+<p>Check-in Friday 12 September after 15:00. Your host Inês will send the door code the evening before.</p>
+<p>Rua da Bica de Duarte Belo 42, Lisboa. The tram stops at the top of the street and the walk down is five minutes.</p>
+<p>Airbnb</p>
+
+--- quoted: none ---
+
+--- text ---
+Check-in Friday 12 September after 15:00. Your host Inês will send the
+door code the evening before.
+
+Rua da Bica de Duarte Belo 42, Lisboa. The tram stops at the top of the
+street and the walk down is five minutes.
+
+Airbnb