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/server/clipboard/manager.go

@@ -1839,21 +1839,34 @@ func (m *Manager) EntryToFile(entry *Entry) string {
 	return ""
 }
 
+func (m *Manager) dbusConnForFlatpak() (*dbus.Conn, error) {
+	m.dbusConnMutex.Lock()
+	defer m.dbusConnMutex.Unlock()
+
+	if m.dbusConn != nil {
+		return m.dbusConn, nil
+	}
+
+	conn, err := dbus.ConnectSessionBus()
+	if err != nil {
+		return nil, fmt.Errorf("connect session bus: %w", err)
+	}
+	if !conn.SupportsUnixFDs() {
+		conn.Close()
+		return nil, fmt.Errorf("D-Bus connection does not support Unix FD passing")
+	}
+	m.dbusConn = conn
+	return conn, nil
+}
+
 func (m *Manager) ExportFileForFlatpak(filePath string) (string, error) {
 	if _, err := os.Stat(filePath); err != nil {
 		return "", fmt.Errorf("file not found: %w", err)
 	}
 
-	if m.dbusConn == nil {
-		conn, err := dbus.ConnectSessionBus()
-		if err != nil {
-			return "", fmt.Errorf("connect session bus: %w", err)
-		}
-		if !conn.SupportsUnixFDs() {
-			conn.Close()
-			return "", fmt.Errorf("D-Bus connection does not support Unix FD passing")
-		}
-		m.dbusConn = conn
+	dbusConn, err := m.dbusConnForFlatpak()
+	if err != nil {
+		return "", err
 	}
 
 	file, err := os.Open(filePath)
@@ -1862,7 +1875,7 @@ func (m *Manager) ExportFileForFlatpak(filePath string) (string, error) {
 	}
 	fd := int(file.Fd())
 
-	portal := m.dbusConn.Object("org.freedesktop.portal.Documents", "/org/freedesktop/portal/documents")
+	portal := dbusConn.Object("org.freedesktop.portal.Documents", "/org/freedesktop/portal/documents")
 
 	var docIds []string
 	var extra map[string]dbus.Variant