ci: Add build check for React todo example

[KyleAMathews]

Summary

• Added a new GitHub Actions job to build the React todo example site
• The check runs pnpm build in the examples/react/todo/ directory
• Build succeeds/fails based on the exit code of the build command

Fixes #188

Test plan

• Verify the new workflow job runs on PRs
• Confirm build failures are properly caught and reported

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9dc7c89

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[pkg-pr-new]

@tanstack/db-example-react-todo

npm i https://pkg.pr.new/@tanstack/db@249

npm i https://pkg.pr.new/@tanstack/db-collections@249

npm i https://pkg.pr.new/@tanstack/react-db@249

npm i https://pkg.pr.new/@tanstack/vue-db@249

commit: 9dc7c89

[github-actions]

Size Change: 0 B

Total Size: 31.4 kB

ℹ️ View Unchanged

Filename	Size
./packages/db/dist/esm/collection.js	8.23 kB
./packages/db/dist/esm/deferred.js	230 B
./packages/db/dist/esm/errors.js	150 B
./packages/db/dist/esm/index.js	528 B
./packages/db/dist/esm/optimistic-action.js	294 B
./packages/db/dist/esm/proxy.js	3.75 kB
./packages/db/dist/esm/query/builder/functions.js	531 B
./packages/db/dist/esm/query/builder/index.js	3.4 kB
./packages/db/dist/esm/query/builder/ref-proxy.js	842 B
./packages/db/dist/esm/query/compiler/evaluators.js	1.34 kB
./packages/db/dist/esm/query/compiler/group-by.js	2.09 kB
./packages/db/dist/esm/query/compiler/index.js	1.42 kB
./packages/db/dist/esm/query/compiler/joins.js	1.15 kB
./packages/db/dist/esm/query/compiler/order-by.js	933 B
./packages/db/dist/esm/query/compiler/select.js	657 B
./packages/db/dist/esm/query/ir.js	318 B
./packages/db/dist/esm/query/live-query-collection.js	2.06 kB
./packages/db/dist/esm/SortedMap.js	1.24 kB
./packages/db/dist/esm/transactions.js	2.29 kB

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[github-actions]

Size Change: 0 B

Total Size: 1.05 kB

ℹ️ View Unchanged

Filename	Size
./packages/react-db/dist/esm/index.js	152 B
./packages/react-db/dist/esm/useLiveQuery.js	902 B

_{compressed-size-action::react-db-package-size}

[samwillis]