feat: prototype Assistant tray prompt and UserModel integration

[Rello]

Motivation

• Provide a small proof-of-concept UI in the tray so users can ask the Nextcloud Assistant from the tray and see the reply inline.
• Reuse the existing OCS taskprocessing connector to schedule text-processing tasks and poll for results instead of hard-coding a new backend path.
• Surface request state, responses and errors in the User model so QML can react to backend progress for a prototype flow.

Description

• Added an OcsAssistantConnector implementation and public API to libsync (src/libsync/ocsassistantconnector.h and src/libsync/ocsassistantconnector.cpp) and wired the sources into src/libsync/CMakeLists.txt to perform the OCS requests (fetchTaskTypes(), fetchTasks(), scheduleTask(), deleteTask()).
• Extended the tray User model (src/gui/tray/usermodel.h / src/gui/tray/usermodel.cpp) with assistant state properties and invokable methods: assistantQuestion, assistantResponse, assistantError, assistantRequestInProgress, submitAssistantQuestion() and clearAssistantResponse(), plus private state and a poll timer (_assistantPollTimer) for polling task results.
• Implemented JSON helper/parsing functions (assistantTaskTypeIdFromResponse, assistantTaskIdFromSchedule, assistantOutputFromTask) and slots to handle connector callbacks: slotAssistantTaskTypesFetched(), slotAssistantTasksFetched(), slotAssistantTaskScheduled(), slotAssistantTaskDeleted(), and slotAssistantRequestError(); these schedule tasks, poll results and update the model state.
• Added a simple tray UI in src/gui/tray/MainWindow.qml (assistantPromptLoader) with a TextField and Button for submitting questions and Labels for showing the assistant response or error, and adjusted layout anchoring so the activity list respects the prompt when visible.

Testing

• No automated tests were executed for these changes.
• The changes were committed locally; CI is expected to run project build/tests after the PR is opened.

---

Codex Task

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-43.zip

Digest: sha256:5ed12477aa0b9f1d7373fafda0316d1984e428393dc9f1a1235728eb7f8cb7b5

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.