Implement CalDAV support

[byquanton]

Current Test Build for commit b3628f8
planify.Devel-x86_64.flatpak

This PR refactors the Planify CalDAV codebase to make it compatible with other CalDAV server implementations besides Nextcloud.

Fixes #1149, #1697

[alainm23]

📋 CalDAV Test Cases - Planify

This document contains test cases for CalDAV support implementation in Planify.

🔧 Initial Setup

Authentication & Connection

Test Case	Nextcloud	Generic CalDAV	Status
AUTH-01: Username/password/URL authentication	N/A	✅	✅
AUTH-02: Handles incorrect credentials	✅	✅	✅
AUTH-03: Nextcloud Login v2 works correctly	✅	N/A	✅

Database Storage

Test Case	Nextcloud	Generic CalDAV	Status
DB-01: Credentials are saved correctly	✅	✅	✅
DB-02: Database structure is correct	✅	✅	✅

📅 Calendar Management

Test Case	Nextcloud	Generic CalDAV	Status
SYNC-01: Lists task-type calendars	✅	✅	✅
SYNC-02: Saves calendars to database	✅	✅	✅
SYNC-03: Detects new/deleted calendars	✅	✅	✅

Task Synchronization

Test Case	Nextcloud	Generic CalDAV	Status
TASK-SYNC-01: Sync token works correctly	❌	❌	❌
TASK-SYNC-02: Detects new/modified/deleted tasks	✅	✅	✅
TASK-SYNC-03: Stores tasks in database	✅	✅	🧪

✅ Task CRUD Operations

Create & Edit Tasks

Test Case	Nextcloud	Generic CalDAV	Status
CRUD-01: Create basic task with title	✅	✅	✅
CRUD-02: Create task with description and due date	✅	✅	✅
CRUD-03: Create task with priority	✅	✅	✅
CRUD-04: Edit task properties	✅	✅	✅
CRUD-05: Complete/uncomplete task	✅	✅	✅
CRUD-06: Delete task	✅	✅	🧪
CRUD-07: Changes sync immediately	✅	✅	✅

🔗 Advanced Features

Subtasks

Test Case	Nextcloud	Generic CalDAV	Status
SUBTASK-01: Create and edit subtasks	✅	✅	✅
SUBTASK-02: Delete subtask	✅	✅	✅
SUBTASK-03: Hierarchy maintained in sync	✅	✅	✅

Task Movement

Test Case	Nextcloud	Generic CalDAV	Status
MOVE-01: Move task between calendars	✅	✅	✅
MOVE-02: Preserve metadata when moving	✅	✅	✅

📊 Test Status Legend

Symbol	Status	Description
✅	Passed	Test passed successfully
❌	Failed	Test failed
❓	Pending	Test is pending execution
🧪	Testing	Test is currently being executed

[byquanton]

I'd love to refine more parts, but won't have much time in the next few weeks/months.

Some sync related bugs that the Nextcloud implementation had, are still there

• Error moving task with subtasks from one projet to another #1496
• Order with Nextcloud task lists out of sync #1457

Except for those bugs, I didn't encounter any other major ones in my testing with Radicale, Baikal & Nextcloud.
For my part this PR is ready for review and can be merged.

[alainm23]

I see the code is quite structured and organized, this is a good starting point for better synchronization with CalDAV. Thank you very much.

[alainm23]

I did some quick tests using Nextcloud, I’ve already updated the test table, just two specific things:

• A migration was done from version 4.13.4 using a Nextcloud account.
• Planify doesn’t detect new task lists when they are created from Nextcloud (When I deleted the account and registered again, the task lists synced correctly).
• The sync token is not being updated each time a change is detected in the projects.

It’s not a blocker for merging the changes; it can be fixed gradually before the release.