feat(dsh-plugin-browserskill): observation overlay — floating card, interrupt, and PiP (Phase 1-B)

Client half of the PiP observation window, wired to the host
ObservationService over the /bsk-observation HTTP+SSE seam:

- ObservationClientStore: initial state fetch + SSE increments +
  on-demand thumbnail blob loading (session-authorized readAttachment,
  same path as the toolview) + interrupt POST. All I/O injectable.
- ObservationOverlay registered into the shell.overlay list seat (the
  sanctioned frame-wide surface; root is off-limits). Hidden with no
  owned sessions; appears on first start; vanishes when all stop.
- Focus view: status row (green/grey/red dot + session + action + mm:ss
  ticking), breathing thumbnail (new frames fade in, failures keep the
  last frame with a warning badge, pre-navigate placeholder), and the
  action area.
- Interrupt: single click, no confirm; greys into 'Interrupting…' and
  back; disabled when nothing is in flight; one-time tooltip explains
  the semantics (stops the current action only; the run continues).
- Card: drag-move via the header, corner-handle resize (min 240x180,
  max 80% viewport), both clamped and kept for the page lifetime;
  collapsible status capsule.
- PiP: Pop out (user gesture) opens Document PiP with the card's
  current size, portals the same content in, and clones the document's
  style nodes; pagehide falls back to the card with state intact;
  unsupported browsers hide the button; nothing auto-pops.

Tests (+17, 89 total): store (fetch/SSE apply/malformed frames/
thumbnail lifecycle/interrupt wire/stop cleanup) and overlay (hidden->
visible lifecycle, status row, thumbnail via loader, interrupt
disabled/active/hint-once, capsule, resize clamps, move clamps, PiP
unsupported, PiP pop/fallback). Styling follows web-styling (dsw-alias
tokens, CSS Modules, focus-visible, reduced-motion).

packages/dsh-plugin-browserskill/package.json

@@ -28,7 +28,8 @@
       "platform": "web",
       "inject": [
         "@deepseek-ai/dsh-client-runtime",
-        "@deepseek-ai/dsh-client-ui-tool"
+        "@deepseek-ai/dsh-client-ui-tool",
+        "@deepseek-ai/dsh-client-ui-layout"
       ]
     }
   },
@@ -75,6 +76,7 @@
     "@deepseek-ai/dsh-attachment": "^0.1.0-rc.6",
     "@deepseek-ai/dsh-client-runtime": "^0.1.0-rc.6",
     "@deepseek-ai/dsh-client-ui-attachment": "^0.1.0-rc.6",
+    "@deepseek-ai/dsh-client-ui-layout": "^0.1.0-rc.6",
     "@deepseek-ai/dsh-client-ui-primitives": "^0.1.0-rc.6",
     "@deepseek-ai/dsh-client-ui-tool": "^0.1.0-rc.6",
     "@deepseek-ai/dsh-llm": "^0.1.0-rc.6",
@@ -83,6 +85,7 @@
     "@testing-library/react": "^16.1.0",
     "@types/node": "^22.10.0",
     "@types/react": "~18.3.1",
+    "@types/react-dom": "^18.3.0",
     "happy-dom": "^15.11.7",
     "lightningcss": "^1.30.0",
     "react": "^18.2.0",