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/server.py

@@ -740,6 +740,7 @@ def update_task(
     description: str | None = None,
     category_id: str | None = None,
     due_date: str | None = None,
+    clear_due_date: bool = False,
     assignee_ids: list[str] | None = None,
     list_id: str | None = None,
 ) -> str:
@@ -756,7 +757,11 @@ def update_task(
             (e.g. ``taskCategory/23431854_200``). Only meaningful for
             shopping lists; ignored for TODO lists.
         due_date: New due date in ISO 8601 format (e.g. ``"2026-04-30T18:00:00"``).
-            Omit to leave unchanged.
+            Omit to leave unchanged. Cannot be used together with *clear_due_date*.
+        clear_due_date: Set to ``True`` to request removal of the due date.
+            **Note:** The Family Wall API does not support clearing a due date once
+            set — this will return an informational error. This parameter exists to
+            surface the limitation explicitly rather than silently doing nothing.
         assignee_ids: New list of member IDs from get_members (e.g. ``["23431898"]``).
             Pass an empty list to remove all assignees. Omit to leave unchanged.
         list_id: Move the task to a different list by providing the target list ID
@@ -766,6 +771,14 @@ def update_task(
     Returns:
         JSON success indicator or an error message.
     """
+    if clear_due_date:
+        return (
+            "Error: Removing a due date is not supported by the Family Wall API. "
+            "The 'dueDate' field cannot be cleared once set — this is a known "
+            "limitation of the API (all tested values are rejected as invalid dates). "
+            "You can set a different future date instead."
+        )
+
     if (
         text is None
         and description is None