feat: AI chat tone selector + graph node pinning

- ai-chat: sélecteur tone (Professional/Créatif/Académique/Décontracté)
  passé via noteContext.tone dans le body vers /api/chat
- network-graph: dragended garde fx/fy → nœud épinglé après drag
  double-clic sur nœud pour désépingler (fx=null, fy=null)
- sprint-status: 6-2 et 6-3 passés en done

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/sprint-status.yaml

@@ -64,8 +64,8 @@ development_status:
   # Epic 6 — Croissance & Activation (PLG) — ajouté 2026-05-29
   epic-6: in-progress
   6-1-onboarding-activation: done             # story-onboarding-activation.md
-  6-2-markdown-roundtrip: review        # brief-markdown-roundtrip.md
-  6-3-brainstorm-canvas-finalize: review        # story: 6-3-brainstorm-canvas-finalize.md
+  6-2-markdown-roundtrip: done        # brief-markdown-roundtrip.md
+  6-3-brainstorm-canvas-finalize: done        # story: 6-3-brainstorm-canvas-finalize.md
   6-4-chat-with-pdf: done                   # already implemented: document-qa-overlay.tsx + document-ingestion + document-search tool
   6-5-pptx-export-watermark: done              # story: 6-5-pptx-export-watermark.md
   epic-6-retrospective: optional