Add collapsible header control to the active plan container and align it with timeline content

[anghelmatei]

Problem

The active plan container takes a lot of vertical space once a plan is generated, and there is no quick way to collapse it while keeping the plan status visible. The container is also slightly misaligned compared to the surrounding chat command/timeline sections, which makes the layout feel off.

Requested change

Add a minimal icon button in the top-right corner of the active plan container that toggles the section between:

• expanded: current full plan body is visible
• collapsed: only a single header row remains visible, showing the existing plan summary line (Plan and Updated ...)

Acceptance criteria

• A small, low-visual-weight expand/collapse control exists in the plan header.
• Clicking it collapses the plan to a single visible row.
• Clicking it again restores the full section.
• The collapsed row keeps the plan context visible (Plan and Updated ...).
• The plan container left alignment matches adjacent chat/timeline sections.
• The control has an accessible label such as Collapse plan / Expand plan.
• Existing plan actions still work and remain visually consistent.
• Collapse state does not flicker or reset during normal rerenders.

Notes

• If the plan row is rendered inside the virtualized timeline, do not keep the collapse state only inside the row component; key it by plan/turn id higher up so it survives remounts.
• This is separate from any existing “expand long plan body” behavior for markdown plans.