chore: stop lint rules that do not model runes from reporting

no-unused-expressions is off for Svelte files: a bare `book;` in an $effect
declares a dependency. The URL and Set instances it flagged are local
temporaries, marked individually. The one real finding was visibleApiKeys,
genuine reactive state cloned on every mutation to force an update; it is a
SvelteSet now.

frontend/eslint.config.js

@@ -41,8 +41,15 @@ export default defineConfig(
 		// no-useless-assignment joined eslint:recommended in ESLint 10, and its flow analysis
 		// does not model runes: it reads `let { ref = $bindable(null) } = $props()` as a value
 		// that is never read. Deleting the default, as it suggests, breaks the binding.
+		//
+		// no-unused-expressions is off for the same reason: a bare `book;` inside an $effect is
+		// how a reactive dependency is declared when the read would otherwise be untracked.
+		// Removing the statement stops the effect re-running.
 		files: ['**/*.svelte'],
-		rules: { 'no-useless-assignment': 'off' }
+		rules: {
+			'no-useless-assignment': 'off',
+			'@typescript-eslint/no-unused-expressions': 'off'
+		}
 	},
 	{
 		files: ['**/*.svelte', '**/*.svelte.ts', '**/*.svelte.js'],