Adds src/shell_client.rs: connects to the WEFT compositor via
WAYLAND_DISPLAY, binds zweft_shell_manager_v1, and calls create_window
for the system UI shell slot (app_id org.weft.system.shell, role panel,
wl_surface null until Servo surface is wired in a later task).
Implements Dispatch for WlRegistry, ZweftShellManagerV1, and
ZweftShellWindowV1. Handles all four window events: configure,
focus_changed, window_closed (calls destroy), and presentation_feedback.
run() in main.rs calls ShellClient::connect() best-effort before
embed_servo; logs a warning if the compositor is not running rather than
propagating the error.
