mozilla / fx-private-relay / 27248121-df67-4761-aaaa-5de5fcf843d0 / 1

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main: 92%

Pull #6345

circleci

build(frontend): upgrade ESLint v8 to v9, fix all violations

Use `ESLINT_USE_FLAT_CONFIG=false` to keep `.eslintrc.js`. Add explicit
`'./src'` pattern, required by v9 legacy config mode.

ESLint v9 can run some plugins that v8 silently ignored.
Fix all newly-enforced violations:

`react-hooks/globals|/refs|/static-components` (React Compiler rules):
* SSR hydration `useState+useEffect` → `useSyncExternalStore`
* `setState` in effects → `startTransition`
* in `render`:
  * components, ref access → moved to module scope
  * `Date.now()` → `useMemo(() => Date.now(), [])` with eslint-disable
  * `Object.assign` instead of mutation

`testing-library/no-container|no-node-access`:
* `container.querySelector()` → `within()`
* `firstChild` checks → `toBeEmptyDOMElement()`

`testing-library/render-result-naming-convention`:
* render result variable "result" → "view"

`@typescript-eslint/no-explicit-any`:
* `any` → typed or `unknown`

`jest-dom/prefer-empty|prefer-in-document|prefer-to-have-attribute`:
* `toHaveTextContent("")` → `toBeEmptyDOMElement()`
* `toBeNull()` → `not.toBeInTheDocument()`
* `getAttribute("class")` → `toHaveClass()`

`prefer-const`
* `let` → `const` where never reassigned

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