refactor(canvas): extract flow reconciliation hook

Move Convex-to-local node and edge reconciliation into a dedicated hook so canvas.tsx has a cleaner sync boundary during modularization. Add hook-level tests for optimistic edge carry and drag-lock behavior to preserve the existing UX.

vitest.config.ts

@@ -12,6 +12,7 @@ export default defineConfig({
     include: [
       "tests/**/*.test.ts",
       "components/canvas/__tests__/canvas-flow-reconciliation-helpers.test.ts",
+      "components/canvas/__tests__/use-canvas-flow-reconciliation.test.tsx",
       "components/canvas/__tests__/use-canvas-sync-engine.test.ts",
       "components/canvas/__tests__/use-canvas-sync-engine-hook.test.tsx",
     ],