core: fix security and concurrency issues found in a backend audit (#2805)

* core: fix security and concurrency issues found in backend audit

Security:
- privesc: pipe the sudo password via stdin (sudo -S) instead of
  embedding it in the command string, so it no longer appears in argv
  (readable by any local user via /proc/<pid>/cmdline or ps)
- greeter: tokenize a session .desktop Exec= line into argv and execve
  directly instead of running it through /bin/sh -c, closing a command-
  injection path via user-writable ~/.local/share/wayland-sessions
- plugins: reject path-separator/.. in plugin id/name before joining
  into a filesystem path, closing an arbitrary-directory-delete in the
  uninstall/update fallback
- keybinds/hyprland: always quote unrecognized bind actions/keys when
  writing generated Lua; only re-emit genuine round-tripped custom Lua
  verbatim (tracked via an explicit flag), closing a Lua-injection path
- desktop/mimeapps: reject newline/bracket in mime/desktop-id fields so
  they can't inject fake sections into the shared mimeapps.list

Robustness / concurrency:
- server: recover panics in the request-dispatch path so one bad
  handler can't crash the daemon and drop every client
- go-wayland: recover panics in the shared dispatch choke point so a
  malformed compositor event can't crash CLI tools / the daemon
- server: per-connection D-Bus client ID instead of a shared constant,
  fixing cross-client signal delivery and subscription teardown
- network: guard the NetworkManager device maps with a mutex (a
  concurrent map read/write here is an unrecoverable fatal error)
- cups: close the event channel on Stop() so Unsubscribe() of the last
  subscriber no longer deadlocks; allocate the fresh channel in Start()
- freedesktop: reuse the shared session conn for the settings watcher
  and tear it down in Close(), fixing a per-Manager conn+goroutine leak
- clipboard: mutex-guard lazy dbusConn creation
- geolocation: use WithMatchMember for the GeoClue2 LocationUpdated
  signal (was WithMatchSender with an interface.member string, so the
  match never fired and live location updates never arrived)
- screenshot: set failed=true on buffer/pool creation errors so the
  dispatch loop doesn't wait forever for a ready/failed that never comes

* apply code review comments

---------

Co-authored-by: bbedward <bbedward@gmail.com>

core/internal/greeter/session_launcher_test.go

@@ -0,0 +1,57 @@
+package greeter
+
+import (
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestParseExecString(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		exec string
+		want []string
+	}{
+		{"plain", "niri --session", []string{"niri", "--session"}},
+		{"extra spaces", "niri   --session", []string{"niri", "--session"}},
+		{"double quoted arg", `env "with space" run`, []string{"env", "with space", "run"}},
+		{"single quoted arg", `env 'with space' run`, []string{"env", "with space", "run"}},
+		{"escaped quote in quotes", `sh "say \\"hi\\""`, []string{"sh", `say "hi"`}},
+		{"field code dropped", "gnome-session %U", []string{"gnome-session"}},
+		{"field code mid-arg", "app --url=%u --run", []string{"app", "--url=", "--run"}},
+		{"literal percent", "app 100%% done", []string{"app", "100%", "done"}},
+		{"shell metachars stay literal", "sh -c $(reboot); echo", []string{"sh", "-c", "$(reboot);", "echo"}},
+		{"empty", "", nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := parseExecString(tt.exec); !reflect.DeepEqual(got, tt.want) {
+				t.Fatalf("parseExecString(%q) = %#v, want %#v", tt.exec, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestExecFromDesktopFileOnlyReadsDesktopEntryGroup(t *testing.T) {
+	t.Parallel()
+
+	path := filepath.Join(t.TempDir(), "example.desktop")
+	writeTestFile(t, path, `[Desktop Action other]
+Exec=/wrong/binary
+
+[Desktop Entry]
+Name=Example
+Exec = /right/binary --flag
+`)
+
+	got, err := execFromDesktopFile(path)
+	if err != nil {
+		t.Fatalf("execFromDesktopFile returned error: %v", err)
+	}
+	if got != "/right/binary --flag" {
+		t.Fatalf("execFromDesktopFile = %q, want %q", got, "/right/binary --flag")
+	}
+}