feat(settings): configure preview content width

[PathGao]

Summary

• Adds a bounded Preview max width control (640–1600 px, default 880 px) with 40 px steppers.
• Clamps manual input on commit and makes the Preview reset action restore the width with its font settings.
• Adds English and Simplified Chinese labels; other languages retain the existing English fallback.

Validation

• npm run check
• npm test (145 passing)
• npm run build

Stack

Part of #263. This PR follows #320, #321, and #322; the next PR adds the scoped keyboard shortcuts and full-width preference migration.

Post-deploy monitoring

• Confirm out-of-range and empty numeric input return to a valid value.
• Confirm a non-English locale can still open and use the control.

[PathGao]

@alecdotdev #323 is the third PR in the #263 stack. It exposes the persisted preview-width preference in Settings with bounds, default guidance, and reset behavior. It follows #320 → #321 → #322.