fix: simplify monaco initialization

[exKAZUu]

Summary

• Replace manual Monaco loader state/effect setup with a cached initialization promise consumed through React 19 use().
• Keep Editor and DiffEditor loading behavior by wrapping their mounted implementations in Suspense fallbacks.
• Make useMonaco hydration-safe for Next.js SSR and update the e2e probe and README guidance.

Why

• React 19 is already the supported peer version, so the loader bridge can be simplified around use() instead of duplicating useState, useEffect, cancellation, and mounting flags.
• The implementation keeps browser-only Monaco initialization out of server rendering to avoid document access during Next static generation.

Testing

• yarn check-for-ai
• yarn typecheck
• yarn test:unit
• yarn test:e2e:next

[gemini-code-assist]

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