feat: add PeopleNameFilter for people-sheet search

Pure case-insensitive display-name match so mesh, geohash, bridge, and
group lists can share one query without duplicating trim/empty rules.

bitchat/Utils/PeopleNameFilter.swift

@@ -0,0 +1,19 @@
+//
+//  PeopleNameFilter.swift
+//  bitchat
+//
+//  This is free and unencumbered software released into the public domain.
+//  For more information, see <https://unlicense.org>
+//
+
+import Foundation
+
+/// Case-insensitive nickname / display-name match for the people sheet search field.
+enum PeopleNameFilter {
+    /// Empty / whitespace-only queries match everything (no filter active).
+    static func matches(_ displayName: String, query: String) -> Bool {
+        let trimmed = query.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else { return true }
+        return displayName.localizedCaseInsensitiveContains(trimmed)
+    }
+}