Feature Request: Add card color coding to Deck

Is your feature request related to a problem? Please describe.

Currently, Nextcloud Deck lacks the ability to color-code cards, which makes it difficult to visually organize and categorize cards by themes, priorities, or project areas. When working with multiple boards or complex projects, users cannot quickly identify which cards belong to specific topic areas or categories at a glance. This limitation reduces the visual organization capabilities that many project management tools offer.

Describe the solution you'd like

Add a card coloring feature that allows users to:

• Assign custom colors to individual cards (e.g., through a color picker or predefined color palette)
• Set colors via the card edit menu or through a quick action (right-click context menu)
• Display colored cards with a colored border, background tint, or colored accent bar
• Optionally create color-coded legends or categories that can be applied to multiple cards
• Filter or sort cards by color to quickly focus on specific themes or categories

The color should be visible in all views (board view, list view, etc.) and remain consistent across the interface.

Describe alternatives you've considered

Current workarounds include:

• Using labels/tags, but these take up visual space and are less immediately recognizable than colors
• Creating separate boards for different themes, but this fragments the workflow
• Using emojis in card titles, but this is not as clean or systematic as proper color coding
• Relying solely on board columns for organization, but this doesn't allow for cross-cutting themes

Additional context

This feature has been requested before in the community forum: https://help.nextcloud.com/t/deck-card-colors/215859
Color-coding is a standard feature in most project management and kanban tools (Trello, Asana, Monday.com, etc.) and would significantly enhance Nextcloud Deck's usability for teams that need to organize cards by multiple dimensions beyond just board columns.

The issue names no files, tests, or entry points, so start by locating Deck's card editing and board/list rendering areas and reviewing the linked community discussion. Before implementation, clarify whether custom colors, legends, filtering, sorting, and all listed views are required; done should be defined by an agreed, testable scope.