[CU-123vqc2bhet] Add files field so pnpm 12 git installs keep build output

[billouboq]

Summary

Add a files field (build, src/index.tsx) to package.json.

pnpm 12 packs a prepared git dependency using the repo's .gitignore when no files field exists. .gitignore lists /build, so only main (build/index.js) survives, and consumers fail on ./katex-script / ./katex-style (beedeez-main web builds: Module not found).

ClickUp

https://app.clickup.com/t/123vqc2bhet

Test plan

Installed as a git dependency in a scratch project:

• pnpm 12.5.1 @ 7f7bf49 (current master): build/ = index.js only.
• pnpm 12.5.1 @ this branch: all 9 build files (index, katex-script, katex-style × .js/.d.ts/.js.map).
• pnpm 10.34.5 @ this branch: all 9 build files (no regression on the current toolchain).

Follow-up in beedeez-main: bump the react-native-katex commit and its allowBuilds depPath key.

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