refactor(frontend): separate session activity state from streaming state (#202)

* loop: refactor-session-activity completed after 1 iterations

* fix(frontend): fix MessagePart and BottomSheet test failures

* refactor(frontend): tighten FloatingTTSButton visibility condition

Use isSessionActive instead of isStreamingResponse to determine when
FloatingTTSButton should be shown, ensuring it only appears when session
is not actively running.

Author: chriswritescode-dev