Add plan versioning and reorganize frontend files

[RohitRajendran]

Summary

• add per-plan schema versioning and migrate legacy floorplans on load
• reorganize components, hooks, stores, utils, and colocated unit tests into per-module folders
• update AGENTS.md and README.md to document the new structure and migration expectations

Verification

• npm run lint
• npm run test -- src/utils/geometry/geometry.test.ts src/utils/storage/storage.test.ts src/hooks/useSnap/useSnap.test.ts src/store/useFloorplanStore/useFloorplanStore.test.ts src/store/useFloorplanStore/undoRedo.test.ts src/store/useToolStore/useToolStore.test.ts src/components/HelpOverlay/HelpOverlay.test.tsx
• npm run build:preview
• npm run test:e2e

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