fix: allow section_header step type in validation, improve tag input

- Add 'section_header' to VALID_STEP_TYPES in backend validation so
  procedural flows with section headers can be published
- Replace procedural editor's inline tag input with TagInput component
  (supports autocomplete, Tab, comma, semicolon, and paste splitting)
- Add semicolon delimiter support to TagInput component

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

backend/app/core/tree_validation.py

@@ -115,7 +115,7 @@ def _validate_children(children: list[dict[str, Any]], path: str, errors: list[d
 
 # --- Procedural Tree Validation ---
 
-VALID_STEP_TYPES = {"procedure_step", "procedure_end"}
+VALID_STEP_TYPES = {"procedure_step", "procedure_end", "section_header"}
 VALID_CONTENT_TYPES = {"action", "informational", "verification", "warning"}