Add shared hashtag normalization helper

js/utils/hashtagNormalization.js

@@ -0,0 +1,50 @@
+// js/utils/hashtagNormalization.js
+
+const HASH_PREFIX_PATTERN = /^#+/;
+
+/**
+ * Normalize hashtags consistently across storage, matching, and UI validation.
+ *
+ * Normalization rules:
+ * 1) Trim surrounding whitespace.
+ * 2) Strip leading "#" characters.
+ * 3) Lowercase the remaining value.
+ *
+ * Note: We do not strip punctuation today. Only add punctuation stripping if
+ * product explicitly decides to change the rules.
+ *
+ * @param {string} value
+ * @returns {string} normalized hashtag or empty string when invalid.
+ */
+export function normalizeHashtag(value) {
+  if (typeof value !== "string") {
+    return "";
+  }
+
+  const trimmed = value.trim();
+  if (!trimmed) {
+    return "";
+  }
+
+  const withoutHashes = trimmed.replace(HASH_PREFIX_PATTERN, "").trim();
+  if (!withoutHashes) {
+    return "";
+  }
+
+  return withoutHashes.toLowerCase();
+}
+
+/**
+ * Format a hashtag for display, ensuring a single leading hash.
+ *
+ * @param {string} value
+ * @returns {string}
+ */
+export function formatHashtag(value) {
+  const normalized = normalizeHashtag(value);
+  if (!normalized) {
+    return "";
+  }
+
+  return `#${normalized}`;
+}