feat(web): floating entity-detail windows (wmkit), replacing sidebar/sheet

Every place that showed entity detail (Knowledge Base's right sidebar,
the EntitySheet drawer used by Knowledge and the chat session graph,
the standalone /entity/:slug page) now opens the entity in its own
floating, draggable, resizable window instead — several can be open
side by side, and clicking a relation inside one opens another,
building up a stack. Windows are managed by one global wmkit instance
(new $lib/stores/windows.ts + $lib/components/EntityDesktop.svelte,
mounted once in App.svelte), themed with the app's own card/border/ring
tokens rather than wmkit's bundled themes (app.css).

- Delete EntitySheet.svelte (redundant) and the KnowledgeBase resizable
  detail pane; row/graph-node click handlers now call
  openEntityWindow(slug) instead of setting local sidebar state.
- SessionGraph (chat's "Scope" mini-graph): clicking a node opens its
  window directly instead of a click-through mini-detail panel with
  its own resize handle and "Full detail" button — that whole
  subsystem is now dead and removed. Node highlight ring is kept
  (still useful to see what you last opened) and now clears itself via
  an effect watching the shared window-manager store, so closing a
  window drops the highlight instead of leaving it pointing at nothing
  — same fix applied to Knowledge Base's row highlight.
- Compact the entity-detail panel's padding (container + each
  DetailSection) now that it's typically viewed in a small window
  rather than a full-height sidebar.
- Fix KnowledgeBase's browse pane losing its flex-1/min-w-0 (and thus
  full width) when the wrapping single-child div around it was removed
  along with the old detail-pane split.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/package.json

@@ -40,6 +40,7 @@
     "vitest": "^2.0.0"
   },
   "dependencies": {
+    "@surdeddd/wmkit": "^0.3.0",
     "clsx": "^2.1.1",
     "d3-force": "^3.0.0",
     "dompurify": "^3.4.11",