Add extended diagnostics logging

- Electron crash handlers (render-process-gone, child-process-gone,
  unresponsive/responsive, process warnings) with a circuit-breaker
  auto-reload for renderer crashes
- Renderer error capture (window.onerror, unhandledrejection, React
  ErrorBoundary) forwarded to the main log via a one-way IPC channel
- Memory-pressure heartbeat measured against the V8 heap_size_limit
- Gated DEBUG log level (RD_DEBUG) and an in-memory ring of recent
  WARN/ERROR lines, exposed via the /errors endpoint and support bundle
- Disk-error classification (ENOSPC etc.) on download failures and
  integrity-check pass/fail logging

tests/error-ring.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from "vitest";
+import { createErrorRing } from "../src/main/error-ring";
+
+describe("createErrorRing", () => {
+  it("keeps entries in insertion order", () => {
+    const ring = createErrorRing(10);
+    ring.push({ ts: "t1", level: "ERROR", message: "a" });
+    ring.push({ ts: "t2", level: "WARN", message: "b" });
+    expect(ring.snapshot().map((e) => e.message)).toEqual(["a", "b"]);
+    expect(ring.size()).toBe(2);
+  });
+
+  it("caps at capacity by dropping the oldest", () => {
+    const ring = createErrorRing(3);
+    for (const m of ["a", "b", "c", "d", "e"]) {
+      ring.push({ ts: m, level: "ERROR", message: m });
+    }
+    expect(ring.snapshot().map((e) => e.message)).toEqual(["c", "d", "e"]);
+    expect(ring.size()).toBe(3);
+  });
+
+  it("snapshot returns a copy, not the live buffer", () => {
+    const ring = createErrorRing(5);
+    ring.push({ ts: "t", level: "WARN", message: "x" });
+    const snap = ring.snapshot();
+    snap.push({ ts: "t2", level: "ERROR", message: "injected" });
+    expect(ring.snapshot().map((e) => e.message)).toEqual(["x"]);
+  });
+
+  it("clear empties the ring", () => {
+    const ring = createErrorRing(5);
+    ring.push({ ts: "t", level: "ERROR", message: "x" });
+    ring.clear();
+    expect(ring.snapshot()).toEqual([]);
+    expect(ring.size()).toBe(0);
+  });
+
+  it("coerces a non-positive capacity to at least 1", () => {
+    const ring = createErrorRing(0);
+    ring.push({ ts: "t1", level: "ERROR", message: "a" });
+    ring.push({ ts: "t2", level: "ERROR", message: "b" });
+    expect(ring.snapshot().map((e) => e.message)).toEqual(["b"]);
+  });
+});

tests/fs-error.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from "vitest";
+import { classifyDiskError } from "../src/main/fs-error";
+import { isDebugFlagEnabled } from "../src/main/logger";
+
+describe("classifyDiskError", () => {
+  it("maps ENOSPC from an error code to a disk-full reason", () => {
+    const err = Object.assign(new Error("write ENOSPC"), { code: "ENOSPC" });
+    expect(classifyDiskError(err)).toMatch(/Festplatte voll/);
+  });
+
+  it("maps EACCES from a code to a permission reason", () => {
+    const err = Object.assign(new Error("nope"), { code: "EACCES" });
+    expect(classifyDiskError(err)).toMatch(/Zugriff verweigert/);
+  });
+
+  it("lower-case codes are normalized", () => {
+    const err = Object.assign(new Error("x"), { code: "enospc" });
+    expect(classifyDiskError(err)).toMatch(/ENOSPC/);
+  });
+
+  it("falls back to scanning the message text when no code is present", () => {
+    expect(classifyDiskError(new Error("operation failed: ENOSPC on volume"))).toMatch(/Festplatte voll/);
+  });
+
+  it("handles a plain string error", () => {
+    expect(classifyDiskError("EROFS: read-only file system")).toMatch(/schreibgeschützt/);
+  });
+
+  it("returns null for an unrelated error", () => {
+    expect(classifyDiskError(new Error("write_drain_timeout"))).toBeNull();
+    expect(classifyDiskError(new Error("premature close"))).toBeNull();
+    expect(classifyDiskError(null)).toBeNull();
+    expect(classifyDiskError(undefined)).toBeNull();
+  });
+});
+
+describe("isDebugFlagEnabled", () => {
+  it("is true for affirmative values", () => {
+    for (const v of ["1", "true", "TRUE", "yes", "on", " on "]) {
+      expect(isDebugFlagEnabled(v)).toBe(true);
+    }
+  });
+
+  it("is false for empty/negative/garbage values", () => {
+    for (const v of [undefined, "", "0", "false", "off", "no", "maybe"]) {
+      expect(isDebugFlagEnabled(v)).toBe(false);
+    }
+  });
+});