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>

README.md

@@ -2,7 +2,7 @@
 
 MCP server for [Family Wall](https://www.familywall.com) -- read and manage your family's circles, lists, and tasks directly from Claude.
 
-## Features (v0.4.14)
+## Features (v0.4.15)
 
 ### Read
 
@@ -16,7 +16,7 @@ MCP server for [Family Wall](https://www.familywall.com) -- read and manage your
 ### Write (with confirmation prompt)
 
 - `create_task` -- create a new task in a list (supports `category_id`, `due_date`, `assignee_ids`)
-- `update_task` -- update text, description, category, due date, assignees, or move to a different list
+- `update_task` -- update text, description, category, due date, assignees, or move to a different list (note: clearing a due date is not supported by the Family Wall API)
 - `toggle_task` -- mark a task complete or reopen it
 - `delete_task` -- permanently delete a task
 - `create_category` -- create a custom category for a shopping list (with optional icon)