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.

src/network/api_client.h

@@ -106,9 +106,17 @@ struct LicenseResult {
 // A 2xx response means the license is valid and the subscription is active.
 // 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.
+// The request also carries the app's version as `appVersion` so the portal can
+// eventually gate accounts on an out-of-date build.
 LicenseResult validateLicense(const std::string& key, const std::string& email,
                               const std::string& systemName = "");
 
+// Builds the JSON body for POST /license. `appVersion` is omitted when empty.
+// Exposed so the request shape can be unit-tested without the network.
+std::string buildLicenseRequestJson(const std::string& key, const std::string& email,
+                                    const std::string& systemName,
+                                    const std::string& appVersion);
+
 // ─── PUT /licenses/{key}/system-name ───────────────────────────────────
 
 // Result of a System Name PUT. `status` is the HTTP status (0 on transport