feat: AI flow builder, visibility model, dashboard tabs, fork UI (#88)

- AI flow builder: scaffold → branch detail → assemble → review flow
- Generate All one-click branch generation with stop/cancel
- Regenerate scaffold suggestions button
- 3-action review screen: Start Flow, Open in Editor, Build Another
- Fix Publish button gated behind !isDirty
- Fix visibility column enforcement in tree access filter
- Add ?visibility filter and author_name to GET /trees
- Dashboard tabbed flows: My Flows / My Team / Public / All
- Create button in My Flows tab, window focus reload (stale data fix)
- Fork UI with optional reason modal
- Fix account_id nullability in User type and schema
- Keep is_public and visibility in sync on updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_ai_tree_validator.py

@@ -124,6 +124,13 @@ class TestReferenceIntegrity:
         errors = validate_generated_tree(tree)
         assert any("non-existent child" in e for e in errors)
 
+    def test_action_next_node_id_references_nonexistent_node(self):
+        """Action next_node_id pointing to a node that doesn't exist anywhere in the tree."""
+        tree = _make_valid_tree()
+        tree["children"][1]["next_node_id"] = "ghost-node"
+        errors = validate_generated_tree(tree)
+        assert any("ghost-node" in e for e in errors)
+
     def test_duplicate_option_ids(self):
         tree = _make_valid_tree()
         tree["options"][0]["id"] = "same"