fix(editor): apply classes to all selected elements

[MelmacAlpha]

Summary

• Apply a class to, or remove it from, every node in a multi-selection via the Selectors panel.
• Preserve single-selection and right-click-outside-selection behavior.
• Keep ambient selectors out of node class assignments.
• Batch the mutation into one undo step while preserving class order and avoiding duplicates.

Why

The Selectors panel previously applied “Apply to selected element” and “Remove from selected element” only to the last-selected canvas node, even when multiple nodes were selected.

Verification

• Targeted tests: 109 passed, 0 failed.
• bun run build passed.
• bun run lint passed.
• Full bun test: 6,828 passed; 11 existing collabRelayIntegration socket-environment tests failed independently of this change.
• No Docker/deployment check required.

Checklist

• Tests cover behavior changes.
• No documentation update is needed for this bug fix; configuration, public APIs, and deployment behavior are unchanged.
• No compatibility shim was added.
• No secrets, local databases, uploads, or generated artifacts are included.

AI disclosure

The implementation, tests, branch preparation, and pull-request preparation were performed by OpenAI Codex under the submitter’s direction. The submitter functionally verified the fix but did not independently review the implementation. Additional maintainer review is requested.

Assisted-by: OpenAI Codex