feat(preview): add width adjustment shortcuts

[PathGao]

Summary

• Adds Cmd/Ctrl+Option/Alt+[ and ] to decrease or increase preview content width by 40 px.
• The shortcuts only run while the preview is visible; inputs, Monaco, modals, Settings, and Home do not receive them.
• Leaving Full width mode is explicit before applying a width adjustment.

Validation

• npm run check
• npm test (147 passing)
• npm run build

Stack

Part of #263. This PR follows #320 → #321 → #322 → #323. The next PR handles the legacy full-width preference migration.

Post-deploy monitoring

• Verify the shortcuts on macOS, Windows, and Linux keyboard layouts.
• Confirm a focused editor, Find input, Settings modal, and confirmation dialog ignore the shortcuts.

[PathGao]

@alecdotdev #324 is the fourth PR in the #263 stack. It adds the scoped Cmd/Ctrl+Option/Alt+[ and ] controls, after #323.