Stream continuous-sensor changes to the live activity log

[iTerminate]

Summary

• LogbookController.handle_state_change (src/hatty/controllers/logbook.py) no longer
  unconditionally suppresses the state_changed fallback while a logbook/event_stream
  subscription is active — it now exempts "continuous" sensors (is_continuous_sensor, the
  issue Activity log shows no history for numeric sensors #29 predicate), since HA's live stream excludes them from its own logbook just like
  logbook/get_events does. A regular entity (light, lock, binary_sensor, …) is still only
  ever appended by the stream, so no double-logging.
• Also drops a no-op change when old_state is available (continuous sensors fire
  state_changed on attribute-only updates too), and renders the entity's unit so a live
  temperature line matches the fetched path's 21.5 °C instead of a bare 21.5.
• ConnectionController._handle_event_message now forwards old_state through to
  log_ctl.handle_state_change.

Fixes #50.

Test plan

• uv run pytest tests/unit/test_logbook_controller.py tests/test_activity_log.py tests/test_dashboard_activity_log.py tests/unit/test_connection_controller.py tests/unit/test_notifications.py
• uv run ruff check .
• uv run pyright
• uv run pytest (full suite, 1310 passed)