fix: surface a00.un API errors + document dueDate-clearing limitation (v0.4.15)

- fw_client: detect nested a00.un errors (previously silent-failed as success)
- update_task: add clear_due_date=True parameter that returns a clear error
  explaining the Family Wall API cannot clear dueDate once set
- SPEC.md: document all tested clearing candidates and their API responses,
  add Fehlerstruktur-Varianten section
- Verified: dueDate cannot be removed via any form-encoded value; all invalid
  date strings are rejected via a00.un.un (silently swallowed before this fix)

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

src/mcp_familywall/fw_client.py

@@ -185,4 +185,12 @@ class FamilyWallClient:
             msg = f"API error from {endpoint!r}: {error_data}"
             raise FamilyWallError(msg, response_data=body)
 
+        # Some endpoints (e.g. taskupdate2) return per-call errors nested under
+        # a00.un.un instead of top-level — detect and surface them.
+        a00 = body.get("a00", {})
+        if isinstance(a00, dict) and "un" in a00:
+            nested = a00["un"]
+            msg = f"API error from {endpoint!r}: {nested}"
+            raise FamilyWallError(msg, response_data=body)
+
         return body