feat: React 19 / Next 16 support (INTER-2317)

[JuroUhlar]

Summary

• Widen react peer range to >=18 <20; add CI job build-and-check-react-19 that overrides the pnpm catalog to React 19 for build/lint/test/test:dts (React 18 stays default).
• Bump examples/next and examples/next-appDir to Next.js 16.2.10 (tsconfig moduleResolution/jsx updated by next build; target cleaned up to es2020).
• Replace eslint-config-next with @next/eslint-plugin-next (the former bundles eslint-plugin-react, incompatible with this repo's ESLint 10), scoped to the two Next examples, with settings.next.rootDir set so its rules actually find pages/app instead of silently no-op'ing.
• Re-add @eslint-react overrides (no-use-context, no-context-provider) for React 19-only idioms, scoped to src//__tests__/.
• Update README requirements to "React 18 or 19"; drop stale "Next 14" reference in examples/next-appDir/README.md.
• Changeset added (minor).

Manual verification

• pnpm build/lint/test/test:dts green under both React 18 and React 19 (CI job's override run locally).
• Manual browser check (real API key, visitor ID resolves, no SDK errors) on all 6 examples under React 18 and 19: next, next-appDir, vite, create-react-app, webpack-based, preact (doesn't use react at all, so version doesn't apply there).
  • Found and fixed along the way: bumping only an example's React version (not the root package) caused a "multiple React copies" crash in CRA/webpack-based,resolved by bumping root too, matching what the CI job's override already does.

Known gap: the CI React-19 job only bumps the root SDK package, not the examples, so example behavior under React 19 is verified manually here but not continuously in CI. Real CI coverage (build/typecheck per example) is a bigger change and will follow in a separate PR.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.84%	111/134
🟡	Branches	67.57%	50/74
🟢	Functions	80.95%	34/42
🟢	Lines	82.31%	107/130

Test suite run success

22 tests passing in 15 suites.

Report generated by 🧪jest coverage report action from ffc711f

Show full coverage report

St	File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
🟢	All files	82.83	67.56	80.95	82.3
🟢	src	88.52	74.07	83.33	88.33
🟢	detect-env.ts	90	83.33	100	90	15,87
🟢	env.ts	100	100	100	100
🟢	env.types.ts	83.33	0	50	83.33	14
🟡	...nt-context.ts	75	100	0	75	8
🟡	get-env.ts	60	0	100	60	8-9
🟢	ssr.ts	100	100	100	100
🟢	...sitor-data.ts	95.65	100	80	95.65	102
🟢	src/components	91.66	58.33	91.66	91.17
🟢	...-provider.tsx	90.32	54.54	90.9	89.65	47,52,85
🟢	...vironment.tsx	100	100	100	100
🟡	src/utils	64.86	69.56	66.66	63.88
🟡	...is-defined.ts	50	75	100	50	3
🟡	to-error.ts	66.66	50	100	66.66	6
🟢	...mise-store.ts	91.3	92.3	100	90.9	20,26
🔴	wait-until.ts	0	0	0	0	8-19

[Copilot]

Pull request overview

This PR updates the SDK and example apps to support React 19 and Next.js 16, adds CI coverage for React 19, and adjusts linting/TypeScript settings to keep the repo green under the new toolchain constraints.

Changes:

• Expand react peer dependency range to >=18 <20 and update README requirements accordingly.
• Upgrade Next.js example apps to Next.js 16.2.10 and apply the TypeScript config changes Next recommends (moduleResolution: bundler, jsx: react-jsx, target: es2020).
• Add a React 19 CI job and scope Next.js ESLint rules to the two Next examples via @next/eslint-plugin-next.

Reviewed changes

Copilot reviewed 9 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Updates documented React requirement to “React 18 or 19”.
package.json	Widens React peer range; adjusts lint to avoid ignored-file warnings; adds @next/eslint-plugin-next.
pnpm-lock.yaml	Locks new Next.js 16 + lint plugin dependency graph.
eslint.config.mjs	Adds Next.js example-only lint rules; ignores regenerated Next next-env.d.ts; reintroduces React 19-related rule overrides for src/ + tests.
examples/next/package.json	Bumps Next.js to 16.2.10 in the pages-router example.
examples/next/tsconfig.json	Aligns TS settings with Next 16 defaults and modernizes compilation target.
examples/next/next-env.d.ts	Updates Next-generated TS env file (now references generated typed-routes file).
examples/next-appDir/package.json	Bumps Next.js to 16.2.10 in the app-router example.
examples/next-appDir/tsconfig.json	Aligns TS settings with Next 16 defaults and includes additional generated type paths.
examples/next-appDir/next-env.d.ts	Updates Next-generated TS env file (now references generated typed-routes file).
.github/workflows/ci.yml	Adds a second CI job that installs React 19 into the workspace for SDK checks.
.changeset/react-19-next-16-support.md	Records a minor release for React 19 / Next 16 support.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

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[github-actions]

🚀 Following releases will be created using changesets from this PR:

@fingerprint/react@3.1.0

Minor Changes

• Add support for React 19 and Next.js 16. The react peer range now accepts >=18 <20. (b539205)

Patch Changes

• Bump @fingerprint/agent dependency to ^4.1.2 (87b776f)

[Copilot]

Pull request overview

Copilot reviewed 10 out of 13 changed files in this pull request and generated no new comments.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file