feat(lungo): add vertically resizable agent chat panel

[misi-bp]

Description

This change improves the lungo main layout so the agent chat can be resized vertically against the workflow graph, similar to the existing horizontal sidebar split.

Behavior

• When the user has sent a message, the main column uses a vertical react-resizable-panels group (graph above, chat below) with a drag handle (ChatPanelSeparator).
• The chat panel starts sized to its content via useChatPanelContentSize, then switches to fill-height so dragging the separator grows or shrinks the scrollable message region.
• Min/max constraints live in chatPanelLayout.ts (e.g. minimum chat height, graph keeps at least ~25% of the column).
• The chat header minimize/maximize control is removed; expanding the conversation is done by dragging the separator instead of toggling collapse in the header.

Refactor

• Shared ResizablePanelSeparator (and resizablePanelLayout constants) extracted from the sidebar separator and reused for both sidebar and chat dividers.

Scope: lungo frontend only (RootPage, ChatArea, layout components/hooks). No API or backend changes.

Issue Link

Fixes #653

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

Manual test plan

1. Open lungo with no messages — chat stays composer-only; no vertical separator.
2. Send a prompt — separator appears; drag up/down to resize graph vs chat.
3. Confirm message thread scrolls when chat is tall and content overflows.
4. Clear conversation — vertical chat panel and separator disappear; graph uses full height.
5. Sidebar horizontal resize still works (shared separator component).

[misi-bp]

This will be updated (rebased) after #712 is merged, conflicts expected.

[pregnor]

LGTM, I will approve after the merge conflict resolution

[delthazor]

lgtm