fix(pi-extension): guard status bar render when ui has no theme (#279)

syncStatus guarded setStatus but used theme.fg unguarded, so a Pi host
exposing setStatus without a theme threw TypeError on session_start (and
agent_start/agent_end/setMode). Require both before rendering.

Add tests for the render path (previously untested) and the theme-absent
degradation. Follow-up to #275 / #84.