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/main.tsx

@@ -1,6 +1,7 @@
 import "@mantine/core/styles.css";
 import "@mantine/notifications/styles.css";
 import "./styles/roulette.css";
+import "./styles/dropzone.css";
 import { MantineProvider, createTheme } from "@mantine/core";
 import { ModalsProvider } from "@mantine/modals";
 import { Notifications } from "@mantine/notifications";