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/packaging/install.sh

@@ -128,6 +128,25 @@ say "Installing ${version:-unknown} from the $channel channel"
 tmp="$(mktemp -d)"
 trap 'rm -rf "$tmp"' EXIT INT TERM
 
+# Tell the app which channel it was installed from. The installer is the only thing
+# that knows, and without this the app kept its own `stable` default and a dev install
+# checked the stable feed — which advertises an OLDER version — reporting "up to date"
+# forever (issue 2183).
+#
+# A plain file rather than a write into the app's SQLite store: shell has no business
+# knowing that schema, and a file it can't misread is the narrowest possible contract.
+# The app reads it at startup (src-tauri/src/update.rs, INSTALL_MARKER) and only acts
+# when the value CHANGED, so switching channel in the app isn't undone on next launch.
+#
+# The directory is Tauri's app-data dir for identifier com.fabledsword.thoughtsync;
+# both sides hardcode it, so a change to the identifier has to change both.
+record_channel() {
+  marker_dir="${XDG_DATA_HOME:-$HOME/.local/share}/com.fabledsword.thoughtsync"
+  # Best-effort: a failure here costs the channel setting, not the install, and a
+  # native install run as root would only be writing into root's home anyway.
+  mkdir -p "$marker_dir" 2>/dev/null && printf '%s\n' "$channel" > "$marker_dir/install-channel" 2>/dev/null || true
+}
+
 # Both native paths install system-wide, so they need root. Resolved once here
 # rather than duplicated per branch; the AppImage path below never calls this.
 need_root() {
@@ -163,6 +182,7 @@ if have pacman && [ -n "$pkg_url" ]; then
   curl -fSL -o "$pkg_file" "$pkg_url"
   need_root
   $sudo pacman -U --noconfirm "$pkg_file"
+  record_channel
   say "Done. Launch ThoughtSync from your application menu, or run thoughtsync."
   native_update_note
   exit 0
@@ -178,6 +198,7 @@ if have dpkg && have apt-get && [ -n "$deb_url" ]; then
   # unconfigured, so `apt-get -f install` is what actually completes that path.
   $sudo apt-get install -y "$tmp/thoughtsync.deb" ||
     { $sudo dpkg -i "$tmp/thoughtsync.deb" || true; $sudo apt-get -f install -y; }
+  record_channel
   say "Done. Launch ThoughtSync from your application menu."
   native_update_note
   exit 0
@@ -233,12 +254,13 @@ have update-desktop-database && update-desktop-database "$apps_menu" >/dev/null
 mkdir -p "$HOME/.local/bin"
 ln -sf "$dest" "$HOME/.local/bin/thoughtsync"
 
+record_channel
+
 say "Installed to $dest"
 printf '    Launch it from your application menu, or run \033[1mthoughtsync\033[0m'
 printf ' (if ~/.local/bin is on your PATH).\n'
-# The AppImage CAN update itself, but the app's own channel is a separate,
-# stable-by-default setting — installing from dev here does not move it.
+# This is the one path where the app can update itself, so say what it will follow.
 if [ "$channel" = "dev" ]; then
-  printf '    You installed from the \033[1mdev\033[0m channel — set the app to match in\n'
-  printf '    Sync → App updates → Development, or in-app updates will follow stable.\n'
+  printf '    In-app updates will follow the \033[1mdev\033[0m channel.'
+  printf ' Change it in Sync → App updates.\n'
 fi