Add a ContextFill widget to the bottom StatusBar showing how full the LLM
context window is (absolute tokens + percent), colored by fill level:
dim < 70%, warning 70-89%, error >= 90%. Updates from stream_end and
compression_started/context_compressed events; seeded from get_session on
resume so the gauge isn't blank pre-turn. None from the backend keeps the
last known value.

Fix: name the widget attribute _ctx_fill, not _context — _context shadows
MessagePump._context (the internal context manager Textual wraps around
message processing) and hangs the app's message pump at mount time.

Polish: drop the "ctx " prefix (shows 12.0k/32.0k · 38%) and add a 1-char
margin between the activity indicator and the gauge.

Co-Authored-By: Claude <noreply@anthropic.com>

Add an animated "agent is working" indicator so a silent gap (long-running
tool, the pause before the first token, sub-agent reasoning) reads as
"alive" rather than "hung": a braille spinner + phase label that runs on
its own timer, independent of the WebSocket event flow.

- New ActivityIndicator widget (rotating spinner + phase label, colored
  status_online), started/stopped from tui_app WS phase transitions
  (thinking / responding / planning / running <tool>) and stopped on
  stream_end/stopped/error and on connection drop.
- New StatusBar bottom line replaces Textual's Footer: indicator on the
  left, dim key-combo summary on the right. One line instead of the menu;
  the key bindings themselves stay on the App so the keys still work.
- Input box: TextArea min-height 3 -> 1, so the prompt is one line by
  default (auto-resize + max-height 12 unchanged).

Co-Authored-By: Claude <noreply@anthropic.com>