fix(editor): prevent Safari slash command crash

[cdskill]

Summary

• Add a Qalma-owned coordinate helper on the editor controller for consumer overlays.
• Use document-position coordinates for sandbox and docs slash command placement instead of DOM Range measurement.
• Remove the temporary marker insertion path that could crash Safari/WebKit when typing /.

Why

Safari could crash when the slash command menu tried to position itself through the DOM selection/range fallback. Moving placement through the mounted editor view's position coordinates avoids mutating the editable DOM during measurement and keeps ProseMirror internals behind a small Qalma API.

Fixes #18

Validation

• nx test editor
• nx test sandbox
• nx test docs
• tsc -p libs/editor/tsconfig.lib.json --noEmit
• tsc -p libs/editor/tsconfig.spec.json --noEmit
• tsc -p apps/sandbox/tsconfig.json --noEmit
• tsc -p apps/sandbox/tsconfig.spec.json --noEmit
• tsc -p apps/docs/tsconfig.app.json --noEmit
• tsc -p apps/docs/tsconfig.spec.json --noEmit
• nx lint editor
• nx lint sandbox
• nx lint docs
• nx build sandbox
• Playwright WebKit: type / in the sandbox editor, menu opens, no crash, no console errors