feat(editor): add optional arrow-key layer reordering

[MelmacAlpha]

Summary

Adds optional ↑ / ↓ shortcuts for moving the selected layer one sibling position within its current parent on the canvas or Layers tree.

• Reuses the existing layers.moveUp / layers.moveDown commands and the moveNode / moveNodes mutation path.
• Adds Settings → Preferences → Layers panel → Move layers with Arrow keys, off by default.
• Adds the new Arrow-key entries to the Settings → Shortcuts reference through the shared keybinding registry.
• Moves multi-selected sibling layers as one ordered block in a single undo step; selections spanning different parents or containing a locked layer remain unchanged.
• Ignores modifier keys, editable fields, inline text editing, and the Alt/Option inspect ladder.
• Documents the preference and shortcut behavior.

Verification

• bun run build
• bun test — not fully green in the local sandbox; environment-dependent failures include temporary-file permission errors and ephemeral-port conflicts, outside this change.
• bun run lint
• Focused regression tests:
  bun test src/__tests__/spotlight/layersCommands.test.ts src/admin/spotlight/__tests__/shortcutDispatch.test.ts src/__tests__/settings/settingsSections.test.tsx src/__tests__/settings/settingsModal.test.tsx src/__tests__/architecture/keybindings-registry-single-source.test.ts

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

Assisted-by: OpenAI Codex

[chatgpt-codex-connector]

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