feat(web): reorderable ServerCards — drag-and-drop + keyboard (#1369) (#1447)

* feat(web): reorderable ServerCards — drag-and-drop + keyboard (#1369)

Let users control the order of servers on the Servers screen instead of
hand-editing mcp.json. Each ServerCard gets a grip handle (in the header,
left of the name) that drives an @dnd-kit sortable; pointer and keyboard
sensors both work, with live-region announcements for screen readers.

- Backend: PUT /api/servers/order rewrites mcp.json in the supplied order
  inside the write lock, reusing the atomic-write + watcher-notify path.
  Registered before /:id so "order" isn't captured as a server id. Rejects
  with 409 when the id set no longer matches disk and 400 on duplicate /
  malformed bodies, so a reorder racing an external edit can't drop or
  duplicate an entry.
- Hook: useServers.reorderServers(orderedIds) — optimistic local reorder,
  PUT, refresh-to-disk-truth on failure.
- UI: SortableServerCard wrapper owns the dnd-kit concerns; ServerListScreen
  wraps the grid in DndContext + SortableContext (rect strategy) with
  pointer + keyboard sensors and announcements. Reorder math and a11y copy
  extracted to reorderIds.ts / serverReorder.ts for direct unit testing.
  Renders plain cards (no grip) when no onReorder is wired.
- Wired onServerReorder through InspectorView -> App.tsx.

Tests: pure-helper units, hook (persist / optimistic / 409 revert), 9
backend integration tests (incl. id-set-mismatch rejection and the
route-registration guard), ServerCard/ServerListScreen units, and a
real-browser Storybook keyboard-reorder play (the dnd-kit keyboard sensor
needs layout rects that happy-dom doesn't provide).

Adds @dnd-kit/core, @dnd-kit/sortable, @dnd-kit/utilities.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(web): notify on reorder failure; skip reorder glue when not reorderable

Address PR #1447 review:
- App.tsx: catch reorderServers rejection (409 from a racing external edit,
  or a network error) and show a red toast, matching the error-handling
  pattern used by every other mutation — previously the rejection was
  discarded with `void` and the drag silently bounced back.
- ServerListScreen: only build handleDragEnd / announcements when
  `reorderable`, so the four-closure announcements object isn't allocated
  every render when no onReorder is wired.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/web/src/App.css

@@ -194,3 +194,17 @@
   max-width: 100%;
   height: auto;
 }
+
+/*
+ * Reorder grip on ServerCard. The grab/grabbing cursor is a draggability
+ * affordance that can't be expressed as a Mantine prop (and the active-press
+ * state is a pseudo-selector), so the whole grab-cursor unit lives here rather
+ * than split across the theme.
+ */
+.server-drag-handle {
+  cursor: grab;
+}
+
+.server-drag-handle:active {
+  cursor: grabbing;
+}