feat(tui): add configurable sidebar width - #34830

Issue for this PR

Closes #6087

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a TUI config option for the session sidebar width:

{
  "sidebar": {
    "width": 50
  }
}

The default remains 42 columns. The session layout and sidebar component now use the same resolved width, and the value is clamped against the terminal width so a large configured value does not consume the full main area.

This also updates legacy TUI config migration so tui.sidebar.width from opencode.json is preserved when moved into tui.json, and documents the option in English and Chinese config docs.

How did you verify your code works?

• cd packages/tui && bun run typecheck
• cd packages/opencode && bun run typecheck
• cd packages/opencode && bun test test/config/tui.test.ts --timeout 30000
• bunx prettier --check packages/opencode/src/config/tui-migrate.ts packages/opencode/test/config/tui.test.ts packages/tui/src/config/index.tsx packages/tui/src/routes/session/index.tsx packages/tui/src/routes/session/sidebar.tsx packages/web/src/content/docs/config.mdx packages/web/src/content/docs/zh-cn/config.mdx
• push hook ran bun turbo typecheck: 29 packages successful

Screenshots / recordings

Not included. This is a config-driven layout change; no visible behavior changes unless sidebar.width is configured.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR