Actually run eslint in CI

[Sbussiso]

The gap

frontend/eslint.config.js exists because npm run lint used to fail with
"couldn't find a config" — its own header comment says the lint command
"had been a no-op for however long. This file fixes that."

It didn't, quite. No CI job has ever run the linter. Frontend audit + build runs npm ci, npm audit, npm test and the production build, and
stops there. So the config landed and nothing invoked it.

Consequences, both concrete:

• 13 eslint errors have been sitting in master with nothing reporting them.
• The eslint 9 → 10 major (Bump eslint and @eslint/js to 10, together #294) had to be evaluated by running the linter
  by hand locally, because CI had no opinion to offer about a linter bump.

Changes

1. deploy.yml gains a Lint (eslint) step in the frontend job, before
the tests.

2. Two rules join the existing warn list in eslint.config.js. They
were the only thing keeping lint at a non-zero exit, and they are the same
family as the four React Compiler advisories this config already demotes,
for the same documented reason:

rule	count	what it reports
react-hooks/immutability	12	useEffect(() => { loadThings() }, [...]) where loadThings is a const declared further down the component
react-hooks/use-memo	1	first argument is not an inline function expression

The immutability one is correct at runtime — effects run after render,
by which point the binding is initialized. It is the React Compiler saying
it cannot track the value, not a live bug. The fix (hoist each loader, or
useCallback) touches six files and belongs with the next edit to each,
not bundled into a CI change.

eslint exits non-zero on errors only, so the 47 deliberate warnings stay
visible in the log without blocking deploys.

Verification

The gate is not vacuous:

$ npm run lint                      # current tree
✖ 47 problems (0 errors, 47 warnings)     → exit 0

$ echo 'export const probe = () => { thisIsNotDefined() }' > src/__lintprobe.js
$ npm run lint
  1:30  error  'thisIsNotDefined' is not defined  no-undef
✖ 48 problems (1 error, 47 warnings)      → exit 1

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