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/plain-text.txt

@@ -0,0 +1,36 @@
+render-version: 2
+message-id: [email protected]
+in-reply-to: -
+references-first: -
+from: Maya Raghunathan <[email protected]>
+to: Priyanshu Jain <[email protected]>
+cc: -
+bcc: -
+reply-to: -
+date-ms: 1788415929000
+subject: Dinner on Thursday?
+is-html: false
+surface: theme
+snippet: Priya said the place on Church Street takes bookings now, want me to put us down for four at eight? Everyone can make Thursday except Karthik, who has the badminton thing and says he will come late…
+list-id: -
+auto-submitted: -
+precedence: -
+unsubscribe: -
+blocked-images: 0
+
+--- html ---
+<p>Priya said the place on Church Street takes bookings now, want me to put us down for four at eight? Everyone can make Thursday except Karthik, who has the badminton thing and says he will come late.</p>
+<p>If you would rather somewhere quieter there is the Malleswaram place we went to in June. Say by tomorrow and I will call them.</p>
+<p>Maya</p>
+
+--- quoted: none ---
+
+--- text ---
+Priya said the place on Church Street takes bookings now, want me to put
+us down for four at eight? Everyone can make Thursday except Karthik, who
+has the badminton thing and says he will come late.
+
+If you would rather somewhere quieter there is the Malleswaram place we
+went to in June. Say by tomorrow and I will call them.
+
+Maya