feat(platform): add server entrypoint with Base44PlatformServer

[ozsay]

Summary

• Restructure packages/platform/src into client/ and server/ entrypoints (each with its own index.ts and modules/); shared origin validation moved to src/shared/server-url.ts.
• Add Base44PlatformServer ({ serverUrl, apiKey }), exposed as @base44/platform/server. No server modules yet.
• package.json: ./server export plus typesVersions for classic node resolution; main entry now dist/client/index.*; typedoc covers both entrypoints.
• Tests: unit tests for the server class; package test checks the tarball, runtime import and declarations under NodeNext/Bundler/Node resolution.

Test plan

• npm test -w @base44/platform (types, unit, typedoc, packed-package checks)

🤖 Generated with Claude Code

[github-actions]

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