Standardize `/share` payload on a structured node object

[devangb3]

• Current state: app.py:156-160, frontend/src/utils/api.js:170-172, frontend/src/components/HomePage.js:307-311, FileExplorer.js:327.
• Proposed implementation:
  1. Expose Node.from_dict(data) (already defined) at the route layer; stop double-encoding.
  2. Change the frontend to send { target, node: <object> } unconditionally.
  3. Remove the typeof node === 'string' ? node : JSON.stringify(node) branch.
  4. Have the server reject non-dict node with a 400.
• Files affected: app.py, frontend/src/utils/api.js, frontend/src/components/HomePage.js, frontend/src/components/FileExplorer.js.
• Acceptance criteria:
  • /share returns 400 for a JSON-string node.
  • All callers send an object.
  • No JSONDecodeError handler remains in the route.