feat(editor): create a notebook from the workspace tab

[NiJingzhe]

Why

The editor could not create a notebook: the "new" button was removed from the header earlier, and the backend endpoint (POST /api/workspace/new, which writes the SDK's template) had no caller. sca edit on an empty directory started and showed nothing.

What

• A new action in the workspace tab's notebooks header opens an inline form: a name, part or assembly, create.
  • The name is checked as you type against the template's rule and the files that exist (bracket.py already exists.); the server's reason is shown if it refuses.
  • An assembly starts with the first two parts of the workspace (the form says which); it is disabled until there is a part.
  • After a successful create the server opens the notebook and the editor shows its cells. After choosing the kind, focus returns to the name field so Enter creates.
• An empty workspace lands on the workspace tab with the form already open, instead of "waiting for the kernel"; the header says what to do.
• The new notebook is the template: PEP 723 header, with app.setup: import simplecadapi as scad (hidden in the editor), a params cell, one feature, the product.

Checked (real editor, browser)

Empty directory: form shown; 9bad name refused; bracket (part) created and opened, tab switched to the notebook; rig (assembly) created, list shows rig ASSEMBLY uses bracket; creating bracket again is refused inline. npm run build passes; the template and creation logic were already covered by test_editor.py.

Not in this PR

sca new <name> [DIR] on the command line (creating the workspace directory and a first notebook without opening the editor).