[FEATURE] Plan mode panel: use available window width on Desktop (currently narrow centered column)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

On Claude Code Desktop with a wide window (e.g., 1600+ px wide on a 1920x1080 or ultrawide monitor), the plan mode panel renders text in a narrow centered column with significant empty space on both sides. Reading and reviewing long plans requires excessive vertical scrolling through a tall, narrow tower of text instead of using the available horizontal space.

This is NOT a terminal width issue — it is the Desktop GUI app, and the constraint persists regardless of window size. It is distinct from the opposite CLI bug (#28326) where chat text stretches too wide on wide terminals.

Proposed Solution

Allow the plan mode panel to use the available window width (no too wide problem, as you can regulate split between chat and preview section)

Ideal behavior: responsive layout where the plan panel grows with the window up to a configurable cap (default could stay at the current narrow width for readability, but power users on wide monitors could raise it).

Alternative Solutions

• Resizing the Desktop window: panel stays narrow regardless of window width.
• Searched settings.json keys and /config: no max-width option for plan panel.
• No environment variable equivalent for Desktop (unlike some CLI options).
• Workaround attempted: none found.

Distinct from related issues:

• #28326 — CLI chat text too wide (opposite problem, terminal-side)
• #23243 — CLI plan text truncation
• #48237 — Desktop font-size request (suggests Desktop layout customization is a gap)

Priority

Low - Nice to have

Feature Category

Configuration and settings

Use Case Example

1. I open Claude Code Desktop on a wide monitor with the window maximized (~1920px+ wide).
2. I ask Claude to plan a non-trivial refactor; it enters plan mode.
3. The plan panel renders as a narrow ~80-char-wide centered column with large empty margins on both sides of the window.
4. Reviewing a long plan (hundreds of lines) requires excessive scrolling and less easy to grasp because the panel does not use the available horizontal space.
5. With this feature, the panel would expand horizontally, reducing scroll length and making side-by-side comparison and review far easier.

Additional Context

• Platform: Claude Code Desktop on Windows 11 Pro
• Setup: wide / maximized window on a wide monitor (3440x1440)
• This is a Desktop-specific layout constraint, not a CLI / terminal issue