desktop: the installer's channel choice now reaches the app (issue 2183)

`install.sh --channel dev` set the channel in the installer and nowhere
else. The app kept its `stable` default, stable advertises 0.1.0, and
0.1.0 is older than any dev build — so every update check said "up to
date", forever, and the user had to know to go set it themselves.

The installer now records the channel as a plain file in the app-data
dir; the app adopts it at startup. A file rather than a write into the
app's SQLite store, because shell has no business knowing that schema.

Adoption compares against the value last adopted, not against "is the
pref unset". Seeding only when unset would have fixed the first install
and left the second silently wrong: install stable, then install dev,
and the pref is already set so dev never takes. Comparing to the last
marker makes both directions work — an in-app channel switch survives
the next launch, and re-running the installer on a different channel is
honoured.

An unreadable marker is ignored rather than read as `stable`, so a
truncated file can't move someone off the channel they're on.

desktop/src-tauri/src/lib.rs

@@ -72,6 +72,9 @@ pub fn run() {
             log::info!("opening local store: {}", db_path.display());
             let db = local::open(&db_path)?;
             log::info!("local store ready — {}", local::summary(&db));
+            // 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);
             sweep_local_trash(&db);
             app.manage(db);
             // Attachment bytes live beside the database, filed by content hash, so a