chore: upgrade ESLint to flat config with dx-team v3

[JuroUhlar]

• Migrate to ESLint 10 flat config with @fingerprintjs/eslint-config-dx-team v3 type-checked rules
• Lint examples from the root config; restore per-example lint scripts
• Apply minimal source/test fixes to pass the stricter rules (no useVisitorData behavior refactor)

Merge plan

We will merge this PR first, then apply additional improvements from #199 on top (notably the useVisitorData refactor and related test coverage).

Discussion point: WithEnvironment render-prop API

WithEnvironment now takes children: (env) => JSX.Element instead of cloning a single child. It is internal-only (not exported from the package entry), but flag if you prefer keeping the old cloneElement shape.

Discussion point: use-visitor-data mount fetch

Kept existing mount getData() behavior with a targeted react-hooks/set-state-in-effect disable and TODO rather than refactoring the hook in this PR.

Note: banner plugin types

vite.config.ts uses @ts-expect-error for rollup-plugin-banner2 and vite-plugin-banner — unmaintained plugins with stale Rollup 4 types, but still required for correct dist output.

Test plan

• pnpm lint
• pnpm test --run
• pnpm build (license banner + "use client" in dist)

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.96% (-0.24% 🔻)	112/135
🟡	Branches	68.42% (+4.65% 🔼)	52/76
🟢	Functions	80.95% (+0.46% 🔼)	34/42
🟢	Lines	82.31% (-0.5% 🔻)	107/130

Show new covered files 🐣

St.❔	File	Statements	Branches	Functions	Lines
🔴	... / assert-is-defined.ts	50%	75%	100%	50%

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / use-promise-store.ts	91.3% (-1% 🔻)	92.31% (+17.31% 🔼)	100%	90.91% (-1.4% 🔻)
🟢	... / fingerprint-provider.tsx	90.32% (-0.85% 🔻)	54.55% (-3.79% 🔻)	90.91% (+0.91% 🔼)	89.66% (-1.25% 🔻)

Test suite run success

22 tests passing in 15 suites.

Report generated by 🧪jest coverage report action from eb3a7a7

Show full coverage report

St	File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
🟢	All files	82.96	68.42	80.95	82.3
🟢	src	88.7	75.86	83.33	88.33
🟢	detect-env.ts	90	83.33	100	90	15,87
🟢	env.ts	100	100	100	100
🟢	env.types.ts	85.71	40	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

Migrates the repo to ESLint 10 flat config using @fingerprintjs/eslint-config-dx-team v3 type-checked rules, updates lint scripts (root + examples), and applies targeted code/test adjustments to satisfy stricter linting without broader behavior refactors.

Changes:

• Add root eslint.config.mjs (flat config) plus tsconfig.eslint.json; simplify root lint script and restore per-example lint scripts.
• Update internal WithEnvironment to a render-prop API and adjust FingerprintProvider + related tests accordingly.
• Apply small source/example/test tweaks to satisfy new TS/ESLint constraints (env detection typing, safer runtime checks, promise handling, etc.).

Reviewed changes

Copilot reviewed 42 out of 45 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
vite.config.ts	Adds TS error suppressions for banner plugins; keeps DTS generation; minor banner string tweak.
tsconfig.json	Expands TS include to cover tests.
tsconfig.eslint.json	Adds a dedicated TS project for type-aware ESLint across src/tests/examples/config files.
src/utils/use-promise-store.ts	Reworks cache key serialization and store initialization approach.
src/utils/use-const.ts	Removes useConst utility (no longer used).
src/utils/assert-is-truthy.ts	Removes truthy assertion helper.
src/utils/assert-is-defined.ts	Adds null/undefined-only assertion helper.
src/use-visitor-data.ts	Switches to assertIsDefined; adds targeted lint disable for mount fetch behavior; minor error logging typing.
src/get-env.ts	Tightens JSON.parse typing to unknown.
src/detect-env.ts	Tightens env-check strategy typing to boolean and adjusts DOM/window checks.
src/components/with-environment.tsx	Changes WithEnvironment from cloneElement to render-prop children API.
src/components/fingerprint-provider.tsx	Adapts provider to new WithEnvironment API; minor refactors/strictness tweaks.
scripts/detect-env.js	Disables @typescript-eslint/no-require-imports for CJS usage.
package.json	Upgrades ESLint + related tooling deps; simplifies lint script.
examples/webpack-based/webpack.config.js	Disables @typescript-eslint/no-require-imports for CJS usage.
examples/webpack-based/package.json	Adds an example-local lint script.
examples/vite/src/vite-env.d.ts	Adds Vite env typing for the example.
examples/vite/src/main.tsx	Adds runtime checks for required env var and root element; fixes import style.
examples/vite/package.json	Restores local lint script; relies on root lint toolchain.
examples/vite/eslint.config.js	Removes per-example flat config (now covered by root config).
examples/preact/src/index.tsx	Adds runtime checks for required env var.
examples/preact/src/components/app.tsx	Marks getData() call as intentionally un-awaited (void).
examples/preact/preact.config.js	Keeps dotenv-based injection in Preact config.
examples/preact/package.json	Adjusts lint script and dependencies for Preact example.
examples/preact/.eslintrc.js	Removes legacy ESLint config (now covered by root flat config).
examples/next/pages/_app.tsx	Adds explicit env var getter/validation.
examples/next/package.json	Switches lint to root ESLint; adjusts dependency placement for a private example.
examples/next-appDir/package.json	Switches lint to root ESLint; adjusts dependency placement for a private example.
examples/next-appDir/app/page.tsx	Removes unnecessary async on page component.
examples/next-appDir/app/layout.tsx	Adds explicit env var getter/validation.
examples/next-appDir/app/HomePage.tsx	Marks getData({}) as intentionally un-awaited (void).
examples/create-react-app/src/shared/pages/SignInPage.tsx	Updates handlers to satisfy stricter linting; simplifies logging of visitor data.
examples/create-react-app/src/shared/pages/HomePage.tsx	Wraps callback bodies to satisfy linting.
examples/create-react-app/src/shared/components/VisitorDataPresenter.tsx	Tightens boolean/undefined checks in rendering logic.
examples/create-react-app/src/shared/components/Toggler.tsx	Updates props typing and wraps callback body to satisfy linting.
examples/create-react-app/src/index.tsx	Adds runtime check for root element presence.
examples/create-react-app/package.json	Adds an example-local lint script.
eslint.config.mjs	New root flat ESLint config wiring dx-team + react + hooks + type-aware parsing.
.eslintrc.js	Removes legacy ESLint config (replaced by flat config).
tests/with-environment.test.tsx	Updates tests to match render-prop WithEnvironment API and modern testing-library patterns.
tests/with-environment.preact.test.tsx	Updates Preact test usage to pass render-prop children.
tests/use-visitor-data.test.tsx	Updates tests to use screen/userEvent patterns and satisfy stricter lint rules.
tests/helpers.tsx	Adjusts typing and wait promise typing to satisfy stricter rules.
tests/fpjs-provider.test.tsx	Removes unnecessary async in test case.

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[github-actions]

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