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/network/api_client.h

@@ -26,9 +26,10 @@ struct HttpResponse {
     bool transportOk = false; // true when the request completed without a transport error
 };
 
-// Performs a GET/POST against `path` (relative to API_BASE_URL).
+// Performs a GET/POST/PUT against `path` (relative to API_BASE_URL).
 HttpResponse httpGet(const std::string& path);
 HttpResponse httpPost(const std::string& path, const std::string& jsonBody);
+HttpResponse httpPut(const std::string& path, const std::string& jsonBody);
 
 // ─── GET /frequencies ──────────────────────────────────────────────────
 
@@ -79,6 +80,10 @@ bool fetchColorPalettes(std::vector<ColorPalette>& out);
 struct LicenseInfo {
     std::string key;
     std::string email_address;
+    // Advisory System Name status returned by the portal: one of "ok",
+    // "unchanged", "conflict", "invalid", or empty when the portal did not
+    // report one (e.g. older portal).
+    std::string system_name_status;
 };
 
 // Parse the 200 JSON payload returned by the license endpoint.
@@ -95,7 +100,25 @@ struct LicenseResult {
 // Validate a license key / email pair against the portal. Never throws; failures
 // (network, HTTP error status) are reported through the returned LicenseResult.
 // A 2xx response means the license is valid and the subscription is active.
-LicenseResult validateLicense(const std::string& key, const std::string& email);
+// When `systemName` is non-empty it is sent as the additive `systemName` field
+// (the Kiosk is the only writer); the response's `systemNameStatus` is advisory.
+LicenseResult validateLicense(const std::string& key, const std::string& email,
+                              const std::string& systemName = "");
+
+// ─── PUT /licenses/{key}/system-name ───────────────────────────────────
+
+// Result of a System Name PUT. `status` is the HTTP status (0 on transport
+// failure); `transport_ok` is false when the request never completed.
+struct PutNameResult {
+    int status = 0;
+    bool transport_ok = false;
+
+    bool ok() const { return status >= 200 && status < 300; }
+};
+
+// Set the System Name for a License Key. 200 on success, 400 invalid_name,
+// 404 unknown_license, 409 name_taken. Never throws.
+PutNameResult putSystemName(const std::string& key, const std::string& name);
 
 // ─── POST /automations/create/account ──────────────────────────────────