Add a "Conversation width" setting (Comfortable / Wide / Full) to Appearance #62

Problem or Motivation

OpenWork renders the chat transcript (and the composer) inside a fixed reading
width — CHAT_LAYOUT.maxWidth = max-w-[840px] (see packages/ui/src/lib/layout.ts,
applied in ChatDisplay.tsx and ChatInputZone.tsx). There is no way for the
user to change it.

On large / ultrawide desktop displays this leaves most of the window empty, and
for code-heavy conversations (wide diffs, long tool output, tables) the 840px
column forces unnecessary wrapping and horizontal scrolling inside blocks.

Comparable desktop clients all expose this as a preference:

• Claude Desktop — "wider chat" width control,
• ChatGPT desktop — chat width setting,
• Codex / VS Code — editor/content width preferences.

OpenWork's otherwise rich Appearance page (theme, color theme, font, language,
rich tool descriptions, reduce motion) is missing this one common affordance.

Proposed Solution

Add a "Conversation width" segmented control to Settings → Appearance →
Interface, right below the existing "Reduce motion" toggle, with three options:

• Comfortable (default) — the current 840px reading column,
• Wide — 1100px, a roomier column for code-heavy chats,
• Full — no max-width, uses the full available panel width.

Behavior:

• Renderer-only preference persisted in localStorage, mirroring the existing
  ReduceMotionContext pattern (no backend/qwen-code change).
• A ConversationWidthProvider reflects the choice onto <html> as
  data-conversation-width and drives a CSS custom property
  --chat-content-max-width (840px / 1100px / none).
• The transcript container and the composer read that variable via an inline
  max-width: var(--chat-content-max-width, 840px), so the fallback keeps the
  shared web viewer (packages/ui SessionViewer) unchanged at 840px.

Feasibility

Frontend-only. Pure renderer/CSS state — the width is a presentation concern
in apps/electron/src/renderer. No qwen-code backend involvement. Follows the
already-merged reduce-motion preference precedent (#50 / #51).

Acceptance criteria (CDP assertion)

A new e2e/assertions/conversation-width.assert.ts drives the real UI over CDP
and asserts, for each of the three options selected from the Appearance page:

1. the segmented control button reports aria-checked="true",
2. <html> carries the matching data-conversation-width value,
3. the computed CSS custom property --chat-content-max-width on <html>
   equals the expected value (840px / 1100px / none),
4. the choice is persisted in localStorage under craft-conversation-width,

cycling through the options to prove it both applies and reverts.