feat: add Gantt chart view for boards

[aparcar]

Summary

Add a timeline/Gantt view as an alternative to the kanban board view, allowing users to visualize card schedules across time using frappe-gantt.

• Add GanttView component with Day/Week/Month view modes
• Add Kanban/Gantt view toggle in board controls
• Store view mode preference in localStorage via Vuex
• Stack-based color coding with legend and undated cards section
• Drag-and-drop support for rescheduling cards
• Auto-fit column width to fill container on wider views
• Add frappe-gantt dependency and webpack resolve alias for its CSS

• Resolves: Add diagramme gantt #134 (an old one!)
• Target version: main

This is inspired by https://github.com/nextcloud-community/ncgantt

TODO

• Sometimes entries "jump" around while dragging, however this might be an issue with my browser

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

[grnd-alt]

works thanks a lot for this contribution. But there is some room for improvement. also darkmode looks like this:

[aparcar]

@grnd-alt I ran Claude again and worked on the fixes. Since this is mostly AI generating (somewhat usable) things, I understand that this quickly starts to waste your time. In other words, please feel free to close this PR until someone comes along with actual web dev knowledge to create a proper implementation.

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[mejo-]

Overall a great addition ❤️

I tested a bit, skimmed through the code changes and made some comments.

[mejo-]

For an empty board, it might be better to switch back to the kanban view, no? When I create a list using this interface, I cannot really do anything with it afterwards, as the gantt view doesn't allow to add another list or cards anyway.

[mejo-]

The whole isDragging and _pendingChange implementation looks a bit overly complex to me. Maybe it's required that way, but since the PR was initially generated by AI, I wonder whether it really is. on_date_change seems to be a callback from the gantt library used here. Why doesn't it work to make it call onDateChange() directly? Why the need to track isDragging and pending changes in Vue explicitly?

[aparcar]

I personally would drop all dragging functionality and just allow the user to click a task and then manually modify the specific dates.

[grnd-alt]

@mejo- I refactored that logic to only have pendingChange, that is required so we only update when the dragging ends and not in between all the time. The logic is now in the global mouseUp listener as the library's on_click seems to not be in the same event chain as the global mouseUp/click so we can't stop propagation between them, and we would have ending drags open the modal, which is quite annoying when you only want to change the date.

[aparcar]

Discussing this with colleagues, they asked for direct links to the gantt or deck view. Could this be added to the url handling?