[board] Modernize kanban board visuals and drag feedback

[frankrousseau]

Problem

• Board columns were heavy grey boxes with no task counts and a stray scrollbar over the header; empty previews rendered as black boxes with a 404 request per card
• Cards left little room to the artwork and drag & drop gave poor feedback (fixed tilt, no drop location preview, no hint when a status requires a preview file)
• Empty columns looked the same whether or not a drop was possible, and the empty-board message only mentioned role configuration

Solution

• Columns become content-hugging panels with a status pill + task count header; columns that cannot receive any task are hidden and the empty state got an illustration and a wording covering all its causes
• Cards show the preview as a full cover background with a translucent scrim, uniform heights, and a placeholder icon when there is no preview
• Drag shows a theme-aware proxy tilting with the drag direction, a ghost slot at the card's real insertion position in the status-tinted target column, and an upload icon when the target status will request a preview file; the ghost holds the pending move while the upload modal is open, which now waits for the upload before closing
• Keyboard focus keeps a rounded :focus-visible ring and prefers-reduced-motion falls back to the native drag