refactor: update deps & code to 2026 toolset

[Lodin]

Summary

This PR updates the project to the current toolchain and removes the remaining legacy setup.

Most of the work falls into four buckets:

• moving tests from Jest/Enzyme to Vitest + React Testing Library
• updating lint/build/typecheck config to the current stack
• fixing Storybook/stories for newer package APIs and typings
• smoothing out source-level compatibility issues exposed by the upgrade

Highlights

• migrated the tree specs to Vitest + RTL
• removed legacy Jest/Enzyme-era config and related setup files
• added modern config for Vitest, tsdown, ESLint, oxlint, and Storybook
• updated stories to work with the current react-virtualized-auto-sizer API
• cleaned up typing issues surfaced by newer React/TypeScript definitions
• refreshed dependencies and lockfile

Notes

This is mostly a modernization / infrastructure PR rather than a behavior change PR.

The biggest risk area is migration fallout from newer typings and dependency APIs, not intentional logic changes. Most source changes are there to keep the codebase, tests, and stories working cleanly in the updated environment.