Bound retained log lines in the live log UI

[paknahad]

Problem

The stream manager now bounds the event channel, but internal/ui.LogModel still appends every rendered log line to m.lines for the lifetime of the session. A long-running or high-volume container can therefore grow terminal UI memory without bound even though the upstream stream buffer is capped.

Proposed scope

Add a focused retention policy for the live log view, such as a configurable or documented maximum number of buffered rendered lines. Keep filtering display-only within the retained window and avoid changing Docker stream behavior.

Acceptance criteria

• The log view caps retained rendered lines at a documented limit.
• Filtering still works over the retained lines without interrupting streams.
• Tests cover retention behavior and ensure newer lines remain visible after the cap is exceeded.

Notes

This should be a UI-buffering change only. Do not combine it with stream manager refactors or Docker adapter changes.