Fix oxlint --deny-warnings and eliminate all biome-ignores

--deny warnings was a no-op (not a valid category); the correct flag
is --deny-warnings. Fixed all 8 pre-existing warnings and removed
every biome-ignore from source and test files. Simplified the check
script to zero-tolerance: any biome-ignore now fails the build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/__tests__/app-integration.test.tsx

@@ -33,8 +33,7 @@ async function addCombatant(
 	opts?: { maxHp?: string },
 ) {
 	const inputs = screen.getAllByPlaceholderText("+ Add combatants");
-	// biome-ignore lint/style/noNonNullAssertion: getAllBy always returns at least one
-	const input = inputs.at(-1)!;
+	const input = inputs.at(-1) ?? inputs[0];
 	await user.type(input, name);
 
 	if (opts?.maxHp) {