fix(client/session): declare the GTK deps the session shell has been using

b0ea1e6b added nine files to clients/session/src/ — app.rs, cli.rs, the four
ui_*.rs, shortcuts.rs, spawn.rs — and did not touch clients/session/Cargo.toml.
The sources `use adw::prelude::*` and `use gtk::{gdk, gio, glib}`; the manifest
declared neither. Every build that compiles this crate has been failing since:
327 errors, all E0433 "cannot find module or crate", taking rpm and deb down with
it. main has been red for both ever since.

Adds a linux-only dependency block — the modules are #[cfg(target_os = "linux")]
in main.rs, and the Windows leg of this crate is a stub that must not pull GTK.
Versions track clients/linux verbatim (gtk4 0.11 "v4_16", libadwaita 0.9 "v1_5",
relm4 0.11 "libadwaita", async-channel 2): the two crates compile the same widget
vocabulary through relm4, where a version skew surfaces as unreadable type
mismatches rather than as a version complaint.

glib/gio/gdk/pango are deliberately NOT declared — the sources reach them as
gtk::glib, gtk::gio, gtk::gdk, gtk::pango, gtk4's own re-exports. Declaring them
separately invites resolving a different version of a sys crate than the one gtk4
links against.

Also makes serde_json non-optional on Linux. It was optional behind the `ui`
feature, but cli.rs's `--json` host listing is a CLI feature, not a console-UI
one — so `--no-default-features`, which this crate's own comments document as a
supported Linux build ("same streaming, stats on stdout only"), did not compile.
That was latent, not part of the CI breakage; found by building it.

Verified on a real GTK4 box (gtk4 4.22.4, libadwaita 1.9.2): cargo check passes
both --no-default-features and default features, the latter being what rpm and
deb build.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

clients/session/Cargo.toml

@@ -41,6 +41,28 @@ anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
+# The GTK4/libadwaita session shell (app.rs, cli.rs, ui_*.rs, shortcuts.rs, spawn.rs). Those
+# modules are `#[cfg(target_os = "linux")]` in main.rs, so their dependencies belong in a
+# linux-only block — the Windows leg of this crate is a stub binary and must not pull GTK.
+#
+# Versions and features track clients/linux verbatim (gtk4 0.11 "v4_16", libadwaita 0.9 "v1_5"):
+# the two crates compile the same widget vocabulary, and a version skew between them would show up
+# as type mismatches through relm4 rather than as anything legible.
+#
+# glib / gio / gdk / pango are NOT listed on purpose — the sources reach them as `gtk::glib`,
+# `gtk::gio`, `gtk::gdk` and `gtk::pango`, gtk4's own re-exports. Declaring them separately would
+# risk resolving a DIFFERENT version of the same sys crate than the one gtk4 links against.
+[target.'cfg(target_os = "linux")'.dependencies]
+gtk = { package = "gtk4", version = "0.11", features = ["v4_16"] }
+adw = { package = "libadwaita", version = "0.9", features = ["v1_5"] }
+relm4 = { version = "0.11", features = ["libadwaita"] }
+async-channel = "2"
+# NOT optional here, unlike the shared block above where it hangs off `ui`. cli.rs's `--json` host
+# listing is a CLI feature, not a console-UI one, so on Linux it is needed whether or not `ui` is
+# on — and `--no-default-features` is a documented Linux build ("same streaming, stats on stdout
+# only"), which without this simply did not compile.
+serde_json = "1"
+
 # Embeds the app icon as an exe resource (build.rs) — Windows hosts only (rc.exe from
 # the SDK); same pattern as clients/windows.
 [target.'cfg(windows)'.build-dependencies]