feat: make chat content width configurable

[uvforce]

Summary

• add a chat content width control under Settings → General → Appearance
• keep the existing 820px width as the default for backward compatibility
• allow 820–2000px in 10px steps and persist the preference in localStorage
• apply one shared CSS variable to existing messages, the empty-session composer, and the regular composer

Behavior

The setting uses a native range input and updates the layout immediately. Narrow windows still shrink below the configured maximum through the existing responsive containers, so the preference does not introduce horizontal page overflow.

The preference is intentionally independent of projects and sessions because it controls the global reading layout.

Testing

• npm test — 847 passed
• node_modules/.bin/tsc --noEmit
• npm run lint
• npm run build
• browser QA:
  • fresh origin loads at 820px
  • changing the setting to 1100px updates and persists both message and composer widths
  • a 900px viewport shrinks content to the available space without horizontal overflow
  • console remained free of errors after reload

AI assistance

OpenAI Codex was used to investigate the existing settings patterns, implement the change, and run the verification above. The final diff was reviewed against the repository guidelines and Issue #681.

Closes #681

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-04T04:19:38.645074Z	8ed117a	PR opened

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