Warning screen, smarter license re-validation, version reporting

Warning screen
- New reusable WarningPage (word-wrapped message, ENTER acknowledges and
  continues to the screen you were about to enter).
- First use: pressing Play while the selected audio is still downloading.
  Dynamic Audio requires every file; a fixed track requires its own file.

License re-validation
- No longer re-verifies on every Home render. Checks now run only when the
  email/license key is entered or changed, on the manual option, at startup
  when already due, and hourly while valid.
- Invalid + unchanged credentials are never re-checked (persisted pair).
- Valid re-checks at most once per hour, anchored on a DB timestamp so an
  overdue check runs immediately after a power cycle.
- New system_kv keys track the checked pair, outcome, and last-check time.
- Added a System Settings row "Manually Validate License Key OK|Invalid"
  (OK green, Invalid red, label white) that runs a check immediately.

Portal / local integration
- License validation request now sends the app version as `appVersion`.
- The entered license key is mirrored to ~/.local/radiantwave/license.txt.

Misc
- "Please connect to WiFi" only shows when no WiFi is joined AND the internet
  is unreachable (fixes the NetworkManager-missing and Ethernet cases).
- Added unit tests for the audio-download warning, license scheduling policy,
  license request shape, and the new settings-menu entry.

tests/test_api_client.cpp

@@ -96,6 +96,41 @@ TEST(ApiClientTest, ParseLicenseJsonRejectsMalformed) {
     EXPECT_FALSE(parseLicenseJson(R"({"key":""})", info));
 }
 
+TEST(ApiClientTest, BuildLicenseRequestJsonIncludesVersionAndSystemName) {
+    const std::string body = buildLicenseRequestJson(
+        "ABCD-EFGH-IJKL-MNOP", "user@example.com", "Living Room", "1.2.3");
+
+    // Round-trips through the parser used for the request body shape.
+    radiantwave::json::JsonValue root;
+    ASSERT_TRUE(radiantwave::json::parseDocument(body, root));
+    EXPECT_EQ(root.stringField("license_key"), "ABCD-EFGH-IJKL-MNOP");
+    EXPECT_EQ(root.stringField("email_address"), "user@example.com");
+    EXPECT_EQ(root.stringField("appVersion"), "1.2.3");
+    EXPECT_EQ(root.stringField("systemName"), "Living Room");
+}
+
+TEST(ApiClientTest, BuildLicenseRequestJsonOmitsEmptyOptionalFields) {
+    const std::string body =
+        buildLicenseRequestJson("ABCD-EFGH-IJKL-MNOP", "user@example.com", "", "");
+
+    radiantwave::json::JsonValue root;
+    ASSERT_TRUE(radiantwave::json::parseDocument(body, root));
+    EXPECT_EQ(root.stringField("license_key"), "ABCD-EFGH-IJKL-MNOP");
+    EXPECT_EQ(root.stringField("appVersion"), "");
+    EXPECT_EQ(root.stringField("systemName"), "");
+    EXPECT_EQ(root.find("appVersion"), nullptr);
+    EXPECT_EQ(root.find("systemName"), nullptr);
+}
+
+TEST(ApiClientTest, BuildLicenseRequestJsonEscapesValues) {
+    const std::string body =
+        buildLicenseRequestJson("k\"ey", "a@b.com", "N\"ame", "1.0");
+    radiantwave::json::JsonValue root;
+    ASSERT_TRUE(radiantwave::json::parseDocument(body, root));
+    EXPECT_EQ(root.stringField("license_key"), "k\"ey");
+    EXPECT_EQ(root.stringField("systemName"), "N\"ame");
+}
+
 TEST(ApiClientTest, ParseCreateAccountJson) {
     const std::string body =
         R"({"success":true,"message":"Account and license key created","user_id":"abc-123"})";