feat: add bidirectional Vikunja task synchronization

[carlos-otero]

Summary

Adds bidirectional task synchronization with Vikunja, allowing tasks to sync between TaskNotes and a self-hosted or cloud Vikunja instance.

Features

• Bidirectional sync: Push tasks to Vikunja on create/update/complete, pull changes from Vikunja on a configurable interval
• Full field mapping: title, status, due date, scheduled date, priority, tags/labels, recurrence, reminders, description (HTML ↔ Markdown), and subtasks (via Vikunja relations API)
• Unified settings UI: Single card in Integrations tab with master toggle that reveals sync options when enabled
• Two-pass pull sync: Resolves race conditions where child tasks are processed before parent tasks exist locally

Field Mapping

TaskNotes	Vikunja
title	title
status	done
due	due_date
scheduled	start_date
priority	priority
tags	labels
recurrence	repeat_after
reminders	reminders
projects	parent task (relations API)
body	description (HTML)

Changes

• src/services/VikunjaService.ts - API client for Vikunja endpoints
• src/services/VikunjaSyncService.ts - Sync logic with two-pass pull, field mapping, HTML/Markdown conversion
• src/settings/tabs/integrationsTab.ts - Unified card UI for Vikunja settings
• src/types/settings.ts - VikunjaSettings interface
• src/settings/defaults.ts - Default settings values
• src/main.ts - Service registration and sync interval scheduling
• README.md - Documentation for Vikunja sync setup

Testing

1. Configure API URL, Token, and List ID in Settings → Integrations → Vikunja Task Sync
2. Enable sync and verify connection with "Test Connection"
3. Create/update tasks in TaskNotes and verify they appear in Vikunja
4. Create/update tasks in Vikunja and trigger "Sync Now" to verify they appear in TaskNotes