desktop: in-app updates follow the server you installed from

Milestone 325 step 6 (Scribe #3254).

The server publishes its AppImage in the updater's own format at
/api/client/linux-appimage/update.json: the ordering key as `version`, the
signature, and an absolute download URL built on the host that was asked,
so the token the updater attaches goes nowhere else. Unsigned platforms and
a server with no AppImage 404.

The desktop's update source is now Fabled-Git (and its channel) or one
server:
- `read_source` is the one reader. The installer's `install-server` marker
  feeds the `update_server` pref once per new value, exactly as the channel
  marker feeds its pref; tauri.conf.json's endpoint is never consulted.
- From a server, the check and the download carry the sync link's token
  when the app is linked to that same server. Without one the update shows
  and says to link rather than offering a button that 401s.
- A server with no build says so. A 404 is "up to date" only on the forge,
  where it means an unpublished channel.
- Sync → App updates offers the source once there is a server to offer (the
  chosen one, or the linked one), and only shows the channel for the forge.

The trust anchor does not move: whatever the source, the updater verifies
the AppImage against the public key built into the app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

desktop/src-tauri/src/lib.rs

@@ -133,6 +133,8 @@ pub fn run() {
             // Before anything can ask what channel we're on: the installer left a note
             // in this directory saying which one the user picked (issue 2183).
             update::adopt_installer_channel(&db, &dir);
+            // And which server, when it installed from one (milestone 325 step 6).
+            update::adopt_installer_server(&db, &dir);
             sweep_local_trash(&db);
             // Before the store is handed to the app: `restore` needs to read the
             // stored shortcut out of it, and after `manage` the Db has moved.
@@ -213,6 +215,7 @@ pub fn run() {
             commands::sync::shares_unshare,
             update::update_channel_get,
             update::update_channel_set,
+            update::update_source_set,
             update::update_check,
             update::update_install,
             capture::capture_shortcut_get,