build(frontend): add ESLint flat config as a gate lane

ESLint 9 flat config, pinned. Narrow by design: the load-bearing rule is
react-hooks (rules-of-hooks + exhaustive-deps) — 33 disable comments existed for a
rule nothing ran. tsc already owns unused vars / undefined / types, so those are
turned off here to report each finding once. no-explicit-any is off: all 19 live in
the api.ts god-module that R7 rewrites, and it flips the rule back on then.

reportUnusedDisableDirectives is an error, which immediately caught two inert
`eslint-disable` comments (App.tsx, AddToPlaylist.tsx) whose effects have stable
deps — removed. no-unused-expressions allows the side-effect ternary idiom the
codebase already uses (`v.paused ? v.play() : v.pause()`).

Errors: 0. Remaining 38 are warnings (28 react-refresh DX, 10 exhaustive-deps) —
pre-existing, visible in the lint output, tracked to their epics.

frontend/src/components/AddToPlaylist.tsx

@@ -37,7 +37,6 @@ export default function AddToPlaylist({
   // its content — hence height — changes, so the flip-above decision uses the true size.
   useLayoutEffect(() => {
     if (open) place();
-    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [open, membership.data]);
 
   function place() {