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>

packages/db/migrations/meta/_journal.json

@@ -22,6 +22,13 @@
       "when": 1772245471347,
       "tag": "0002_numerous_siren",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "7",
+      "when": 1772312586761,
+      "tag": "0003_motionless_peter_parker",
+      "breakpoints": true
     }
   ]
 }