fix(preview): migrate full-width preference

[PathGao]

Summary

• Moves Full width persistence from the generic isFullWidth key to preview.fullWidth.
• Preserves existing users’ setting when the new key is absent; a valid new-key value wins when both exist.
• Removes the legacy key after the current state is persisted.

Validation

• npm run check
• npm test (149 passing)
• npm run build
• cargo test (25 passing)

Stack

Closes #263. This PR follows #320 → #321 → #322 → #323 → #324.

Post-deploy monitoring

• Verify current Full width users retain their selection across the first upgraded launch.
• Verify a changed Full width selection persists to the dedicated key.

[PathGao]

@alecdotdev #325 is the fifth PR in the #263 stack. It migrates the legacy full-width preference while preserving existing users’ selection. It follows #324.

[PathGao]

@alecdotdev All five checks are now green on #325. The #263 stack is ready in order: #320 → #321 → #322 → #323 → #324 → #325; merging #325 will close #263 automatically.