feat(client): live per-session activity dashboard in the Flutter client sidebar

[emal-avala]

Summary

Turns the Flutter client's plain session list into a live agent dashboard: each session shows an at-a-glance status, plus a summary header, so you can see across all your sessions which are busy or waiting on you — the graphical counterpart to the terminal client's tasks view.

What changed

• SessionActivity enum (label, glyph, sort order) mirroring the terminal client's task states, plus an Idle for connected-but-quiet sessions.
• SessionBloc derives activity per session from that session's WebSocket streams — no dependency on the active chat:
  • tool_start / text_delta / thinking → Working
  • incoming ask_permission → Needs input
  • done / turn_complete → Idle
  • error → Failed
  • Subscriptions are tracked per session and cancelled on destroy/close (no leaks). WsClient creation is injectable purely so tests can drive controllable streams; production behavior is unchanged.
• SessionState carries Map<String, SessionActivity> (activityFor helper), and copyWith gains a clearActive flag — fixing a latent bug where the active session id could never be set back to null (destroying the last session now empties it).
• Sidebar: a themed status dot + label per session, and a summary header (N need input · M working · K idle, zeros omitted, "All idle" when calm), surfacing needs-input/working first.

Verification

• 46 tests pass (flutter test): SessionActivity, SessionBloc activity derivation from mocked WS streams (working/needs-input/idle/failed transitions, subscription cleanup, late-notification safety), and Sidebar widget rendering (labels, dots, summary counts, empty state).
• flutter analyze clean for the new code (remaining infos are pre-existing prefer_const_constructors in unrelated test files).
• Built to web and rendered in a real browser via Playwright (system Chrome) — the app boots and renders without console errors; screenshot reviewed. Live status badges need a running agent serve backend the web client connects to; their rendering is covered by the widget tests.
• Adds client/e2e/dashboard-shot.spec.ts — a Playwright screenshot spec for visual review of the running web client.

Notes

• This is the dashboard core (per-session live state). Natural follow-ups: surface background subagents/tasks within a session, a fuller overview/grid view, and richer chat (tool cards + inline diffs mirroring the terminal client).

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