Compact task cards: assignee where the id was, color-only indicators, no divider

[gnacho]

Make the desktop task card more compact:

• Remove the short id from the card; show the assignee avatar in that spot (top right of the project row) when the task is assigned.
• Remove the horizontal line that separates the title from the bottom row.
• In the indicator row, drop the text: priority and due date become color-only dots (priority and overdue/today keep their colors, tooltips keep the text accessible).
• Show just the paperclip icon when the task has attachments (no count).