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.

flatpak/studio.margin.mail.metainfo.xml

@@ -0,0 +1,37 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>studio.margin.mail</id>
+  <name>Margin Mail</name>
+  <summary>A calm, keyboard-first mail client for Gmail</summary>
+
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>LicenseRef-proprietary=https://github.com/priyanshujain/margin-mail/blob/main/LICENSE</project_license>
+
+  <developer id="studio.margin">
+    <name>Margin</name>
+  </developer>
+
+  <description>
+    <p>
+      New senders wait at the door until you let them in, people and newsletters and receipts live
+      in three separate boxes, and every decision you make is kept beside the mail rather than
+      inside it.
+    </p>
+    <p>
+      Trackers are stripped from what you read and never sent from what you write. The mirror of
+      your mail and every decision you have made about a sender are files on your own disk, so
+      changing provider costs you nothing.
+    </p>
+  </description>
+
+  <launchable type="desktop-id">studio.margin.mail.desktop</launchable>
+  <url type="homepage">https://github.com/priyanshujain/margin-mail</url>
+  <url type="bugtracker">https://github.com/priyanshujain/margin-mail/issues</url>
+
+  <categories>
+    <category>Office</category>
+    <category>Email</category>
+  </categories>
+
+  <content_rating type="oars-1.1"/>
+</component>