Add arrow key navigation between expandable sections (ctrl+o)

Feature Request

Problem

When a conversation contains multiple expandable sections, selecting a specific section for expansion via ctrl+o is difficult. There's no way to quickly navigate between expandable sections.

Proposed Solution

After pressing ctrl+o to expand a section, allow Up Arrow and Down Arrow to navigate to the previous/next expandable section in the conversation. This would let users quickly jump between sections without having to manually scroll and precisely select each one.

Expected Behavior

1. Press ctrl+o to enter section expansion mode
2. Press Down Arrow to move focus to the next expandable section
3. Press Up Arrow to move focus to the previous expandable section
4. Press Enter (or ctrl+o again) to expand/collapse the focused section

Motivation

In conversations with many expandable sections (e.g., tool call results, long code blocks), it's cumbersome to target a specific section for expansion. Arrow key navigation would make this interaction much more fluid.