Include desktop app logs in diagnostics (#1914)

* fix(diagnostics): include desktop app logs

Settings diagnostics now collect the Electron main-process log separately from daemon.log so desktop startup and environment-probe failures are included in app reports.

* refactor(desktop): share log tail reader

* fix(diagnostics): surface desktop app log read errors

* refactor(diagnostics): tighten desktop report API

packages/app/src/diagnostics/app-diagnostic-report.test.ts

@@ -82,6 +82,7 @@ describe("app diagnostics report", () => {
     const host = makeHost();
     const redacted = redactAppDiagnosticReport(
       [
+        "Desktop app log tail",
         "secret.example.test:6767",
         "relay.secret.test:443",
         "daemon-public-key-secret",