Kiosk: System Name storage, validation, sync, and sent on license validation

Adds local System Name persistence (trimmed, 1-40) in system_kv, an
injectable sync module with conflict/invalid revert and a pending-sync
flag, a background SystemNameSync worker (retries on boot and on a timer),
and api_client support for PUT /licenses/{key}/system-name plus the
additive systemName field/systemNameStatus on license validation. The
license service sends the stored name and applies the status. Covers
validation, pending retry, and conflict revert with a faked HTTP layer.

src/validation.cpp

@@ -87,4 +87,21 @@ std::string formatLicenseKey(const std::string& sanitized) {
     return out;
 }
 
+std::string trim(const std::string& input) {
+    size_t start = 0;
+    size_t end = input.size();
+    while (start < end && std::isspace(static_cast<unsigned char>(input[start]))) {
+        ++start;
+    }
+    while (end > start && std::isspace(static_cast<unsigned char>(input[end - 1]))) {
+        --end;
+    }
+    return input.substr(start, end - start);
+}
+
+bool isValidSystemName(const std::string& name) {
+    const std::string trimmed = trim(name);
+    return !trimmed.empty() && trimmed.size() <= SYSTEM_NAME_MAX_LENGTH;
+}
+
 } // namespace radiantwave::validation