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/desktop/src/diagnostics/tail-file.test.ts

@@ -0,0 +1,34 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { tailFile } from "./tail-file";
+
+let testDir = "";
+
+describe("tailFile", () => {
+  beforeEach(() => {
+    testDir = mkdtempSync(path.join(tmpdir(), "paseo-tail-file-"));
+  });
+
+  afterEach(() => {
+    rmSync(testDir, { recursive: true, force: true });
+  });
+
+  it("returns the requested tail lines", () => {
+    const filePath = path.join(testDir, "main.log");
+    writeFileSync(filePath, "one\ntwo\nthree\n");
+
+    expect(tailFile(filePath, 2)).toBe("two\nthree");
+  });
+
+  it("keeps missing files empty", () => {
+    expect(tailFile(path.join(testDir, "missing.log"), 2)).toBe("");
+    expect(tailFile(path.join(testDir, "missing.log"), 2, { throwOnReadError: true })).toBe("");
+  });
+
+  it("can propagate read failures", () => {
+    expect(() => tailFile(testDir, 2, { throwOnReadError: true })).toThrow();
+    expect(tailFile(testDir, 2)).toBe("");
+  });
+});