chore: tighten lint rules and tests

src/lib/normalize-handle.ts

@@ -1,13 +1,21 @@
+const HANDLE_REGEX = /^[A-Za-z0-9_]{1,15}$/;
+
 export function normalizeHandle(input?: string | null): string | null {
   const raw = (input ?? '').trim();
-  if (!raw) return null;
+  if (!raw) {
+    return null;
+  }
 
   const withoutAt = raw.startsWith('@') ? raw.slice(1) : raw;
   const handle = withoutAt.trim();
-  if (!handle) return null;
+  if (!handle) {
+    return null;
+  }
 
   // X/Twitter handles are traditionally max 15 chars; keep strict to avoid surprising queries.
-  if (!/^[A-Za-z0-9_]{1,15}$/.test(handle)) return null;
+  if (!HANDLE_REGEX.test(handle)) {
+    return null;
+  }
 
   return handle;
 }
@@ -16,7 +24,9 @@ export function mentionsQueryFromUserOption(userOption?: string | null): {
   query: string | null;
   error: string | null;
 } {
-  if (typeof userOption === 'undefined') return { query: null, error: null };
+  if (typeof userOption === 'undefined') {
+    return { query: null, error: null };
+  }
 
   const handle = normalizeHandle(userOption);
   if (!handle) {