feat(kanban): drag-aware sidebar dropzone (issue 0091)

Add a 32px invisible strip on the left edge of the viewport that
auto-opens the sidebar when the user drags a card and dwells near the
edge for >=400ms. Removes the manual toggle step when moving cards to
sidebar-located columns.

- App.tsx: global mousemove listener while drag is active; 400ms hover
  timer; sets navOpen(true) when triggered; cancels on pointer leave or
  drag end. No auto-close on drag end (user keeps sidebar open).
- dropzone.css: subtle inset blue glow with pulse animation while
  pointer is inside the strip and a drag is active.
- KanbanColumn.tsx: add data-column-id and data-column-location to the
  Paper root for stable e2e selectors.
- e2e/sidebar-dropzone.spec.ts: Playwright test driving a slow drag
  to the left edge, asserting the strip arms, sidebar opens, and the
  card moves to a sidebar column via /api/board.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/KanbanColumn.tsx

@@ -261,6 +261,8 @@ function KanbanColumnImpl({
       withBorder
       radius="md"
       p="sm"
+      data-column-id={column.id}
+      data-column-location={column.location}
     >
       <Group justify="space-between" mb="xs" wrap="nowrap">
         <Group gap={4} wrap="nowrap" style={{ flex: 1, minWidth: 0 }}>