feat: implement Stories 3.4, 3.5, 3.6 — AI proposals, wizard, hover & palette

Story 3.4: AI semantic suggestions with accept/reject workflow
- ProposalBar overlay with visual diff
- Accept/reject flow with graph snapshot restore
- useProposalDiff hook for change summary
- System prompt scoping for selected elements

Story 3.5: New diagram wizard with AI type inference
- CreateDiagramDialog with AI type inference (Haiku)
- initialDescription prop for chat-first flow
- Auto-send on mount with hasSentInitial ref guard
- DB migration for diagram description column

Story 3.6: Hover affordances and command palette
- HoverAffordances toolbar (5 AI actions, debounced)
- CommandPalette (Cmd+K) with AI, nav, Go to Node
- prefillChat/fitViewRequested/focusNodeId actions
- Code review: getNodesBounds, onOpenRightPanel,
  timer cleanup, test count fix

374 tests passing (251 web + 123 AI).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -66,9 +66,9 @@ development_status:
   3-1-chat-panel-ui-with-streaming-ai-responses: done
   3-2-ai-diagram-generation-from-natural-language: done
   3-3-badge-based-element-referencing-for-targeted-modifications: done
-  3-4-ai-semantic-suggestions-and-accept-reject-workflow: backlog
-  3-5-new-diagram-wizard-with-ai-type-inference-and-chat-first-onboarding: backlog
-  3-6-hover-affordances-and-command-palette: backlog
+  3-4-ai-semantic-suggestions-and-accept-reject-workflow: done
+  3-5-new-diagram-wizard-with-ai-type-inference-and-chat-first-onboarding: done
+  3-6-hover-affordances-and-command-palette: done
   epic-3-retrospective: optional
 
   # ── Epic 4: Real-Time Collaboration (Phase 2) ──