feat: Kanban UX upgrade — richer cards, side panel, configurable columns

[Allann]

Summary

• Visual overhaul of the kanban board inspired by the as-notes design: priority chips with semantic colour coding (critical/high/normal/low), column accent top-border colours, assignee initials avatars, tag pills with overflow count, and empty-column drop-zone placeholders
• Task side panel replaces the inline modal — slides in from the right when a card is clicked, stays visible while browsing the board, auto-focuses the title, shows a live tag pill preview, relative timestamps (created/moved), and disables Save until changes are made
• Drag-and-drop improvements: columns highlight on drag-over, cards fade/scale while being dragged, dragLeave uses relatedTarget containment to prevent flicker
• + Add task on every column (previously only Backlog) using a dashed affordance button
• Done column time-window filter: defaults to last 7 days, cycles through 7 / 14 / 30 / all via a pill button in the column header; hidden task count shown as a "show more" button
• Configurable columns: add, rename (double-click), and delete columns inline — Backlog and Done are protected; delete is disabled while a column has tasks

Backend changes

• BoardColumn.Rename() — mutable title
• Board.AddColumn / RenameColumn / RemoveColumn with protection and validation
• IBoardService / BoardService — three new async methods
• API: POST /api/board/columns, PATCH /api/board/columns/{id}, DELETE /api/board/columns/{id}
• StudioApiClient — three new methods + patchJson helper
• Protocol — addColumn / renameColumn / deleteColumn message types
• KanbanPanelProvider — routes the new messages

Test plan

• Drag cards between columns — drop zone highlights, card fades while dragging
• Click a card — side panel slides in, Escape closes it, clicking another card switches it
• Edit task fields — Save enables only after a change; saving clears the "unsaved" label
• Done column — confirm only last 7 days shown by default; cycle pill through 7d → 14d → 30d → all
• Add a column via the + header button
• Double-click a non-protected column title to rename; press Escape to cancel
• Delete an empty non-protected column; confirm button is disabled when column has tasks
• Confirm Backlog and Done have no rename or delete controls

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