feat: Object sidebar — file upload, notes, tags, tasks & component refactor

[rubenvdlinde]

Summary

Rework the CnObjectSidebar to fix broken functionality and add missing features across all tabs (Files, Notes, Tags, Tasks, Audit Trail), then refactor each tab into a self-contained component.

Changes

Bug Fixes

• File upload: Fixed broken upload button (NcButton inside label swallowed clicks), replaced with drag-and-drop zone
• File upload endpoint: Changed from /files (expects JSON name+content) to /filesMultipart (accepts FormData)
• Notes creation: Fixed missing setVerb('comment') causing "Actor, Object and Verb" error
• Tags: Reworked from broken object-property approach to Nextcloud system tags

New Features

• File drop zone: Drag-and-drop + click-to-browse with visual feedback and error display
• Notes editing: Edit/delete actions via dropdown menu, inline editing with save/cancel
• Object-level tags: New API endpoints using Nextcloud system tags with autocomplete suggestions
• Task creation: Add task form with summary, deadline date picker, and user assignee select
• Task actions: Complete and delete tasks from the sidebar
• Auto-create calendar: TaskService creates a VTODO calendar if none exists

Refactor

• Extracted each tab into its own component: CnFilesTab, CnNotesTab, CnTagsTab, CnTasksTab, CnAuditTrailTab
• CnObjectSidebar is now a thin shell (~100 lines)
• All tab components are independently importable
• Slot overrides remain backwards compatible

Repos Affected

• openregister — Backend API endpoints (tags, notes, tasks, calendar)
• nextcloud-vue — Frontend tab components

Test Plan

• Upload files via drag-and-drop and click-to-browse
• Add, edit, and delete notes
• Add and remove tags (with autocomplete)
• Create tasks with summary, deadline, and assignee
• Complete and delete tasks
• Verify audit trail tab loads