fix(dashboard): skip stale canvas deletes - #7334

Summary

• preserve canvas node and edge array references when cascade deletion finds no matching node
• skip undo history and state updates when a context-menu node or connection target has gone stale
• cover stale node and connection deletion in the canvas helper tests

Verification

• corepack pnpm exec vitest run src/lib/canvas.test.ts (1 file, 9 tests)
• corepack pnpm test (101 files, 1061 tests)
• corepack pnpm lint
• corepack pnpm typecheck
• corepack pnpm build

Out of scope

• none