Perf/panel memory cleanup - #13

- HLSPlayer.pause/resume stop segment download (hls.stopLoad) and
  video decode when the TV panel is hidden; skips pause if video is
  in PiP so the picture-in-picture keeps playing
- TVPanel wires onVisibilityChange to the new pause/resume methods
- TasksPanelRuntime also calls detailCleanup() on hide so the Docker
  3s poll stops alongside the diff-refresh interval

When the Tasks panel is hidden the Docker 3s poll now stops cleanly;
when it becomes visible again resumePoll() fires an immediate refresh
and restarts the interval — no user interaction required.

- Terminal scrollback 10 000 → 2 000 lines (5x less RAM per terminal;
  2 000 is still plenty for typical debug sessions)
- HLS.js maxBufferLength 30s → 10s, maxMaxBufferLength 600s → 20s
  (cuts pre-download buffer for TV streams by 3x)

…n hidden

- WebPanel now tracks dockview visibility via onVisibilityChange so the
  native webview overlay is hidden immediately when another tab is active,
  preventing it from painting over other panels (visibility:hidden from
  dockview does not trigger IntersectionObserver)
- lazyPanel queues the last onVisibilityChange call and replays it once
  the inner module finishes loading, so TV/terminal panels that are
  hidden before their import() resolves don't start streaming/running

…rics

…n dv-content-container cells

…nel with + button

… flex height resolution

… when 2+ open in same group

Clicking either member of a split pair no longer destroys the split —
it just focuses that terminal. The pair is visually connected in the
sidebar with a left accent border. Removing a group member clears the
split and adjusts the secondary index after the splice.

Right-click now shows 4 directions (←→↑↓). Each adds a new terminal
relative to the one under the cursor. Adding in the same direction as
the existing split extends the group; adding perpendicular clears the
old split and starts a new one. Closing a member shrinks the group
gracefully. CSS uses flex order to reposition members without DOM
reorder, so position:absolute non-members stay out of flex flow.

Adding a new agent via the sidebar footer no longer destroys the active
split — the new agent is registered in the list and the user can switch
to it or split from it from there. Also re-renders sidebar after each
directional split to refresh group member indicators.