Replace the ad-hoc Node `__eq__` in share dedup with a stable node ID

[devangb3]

ShareManager.receive uses node in self.share_list[from_username], which relies on Node.__eq__ comparing name/is_folder/file_obj. Two different files named "report.pdf" from the same sender with different CIDs will be treated as identical because Node.__eq__ doesn't actually check child structure on folders (children dict isn't compared). A stable per-node UUID fixes multiple bugs at once.

Current state:
Node.__eq__ in node.py compares (name, is_folder, file_obj); children and subtree are not part of equality. File.__eq__ compares all four fields including creation_date, so it's narrower than expected.

Proposed implementation:

1. Add node.id = str(uuid.uuid4()) on creation, persisted in JSON.
2. ShareManager.receive dedups by id.
3. ShareManager.delete takes an id instead of comparing by (name, is_folder).
4. Migration: on load, backfill missing IDs.

Files likely affected:

• backend/node.py
• backend/share_manager.py
• backend/delete_service.py
• Tests.

Acceptance criteria:

• Two distinct files with the same name from the same sender are both kept in the share list.
• Deleting one removes exactly one.
• Existing serialized trees continue to load.