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/email_address_page.cpp

@@ -92,6 +92,8 @@ void EmailAddressPage::submit()
             storedValue_.clear();
             statusMessage_ = "Cleared";
             returnTimer_ = 1.0f;
+            // The credential pair changed: re-validate in the background.
+            state_->licenseTracker.refresh();
         } else {
             statusMessage_ = "Failed to clear email address";
         }
@@ -107,6 +109,8 @@ void EmailAddressPage::submit()
         storedValue_ = input_;
         statusMessage_ = "Saved";
         returnTimer_ = 1.0f;
+        // The credential pair changed: re-validate in the background.
+        state_->licenseTracker.refresh();
     } else {
         statusMessage_ = "Failed to save email address";
     }