sanitizeName: allow forward slash in display names

Sideband/Columba and other LXMF clients commonly include "/" in display
names (e.g. handle/path conventions). The allowlist in sanitizeName
stripped it, mangling "alice/columba" into "alicecolumba". Saved
contacts get re-sanitized on each load (line 393), so existing names
also lose the slash on next boot.

src/reticulum/AnnounceManager.cpp

@@ -82,7 +82,7 @@ static std::string sanitizeName(const std::string& raw, size_t maxLen = 16) {
     for (char c : raw) {
         if (clean.size() >= maxLen) break;
         if ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
-            (c >= '0' && c <= '9') || c == ' ' || c == '-' || c == '_' || c == '.' || c == '\'') {
+            (c >= '0' && c <= '9') || c == ' ' || c == '-' || c == '_' || c == '.' || c == '\'' || c == '/') {
             clean += c;
         }
     }