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

src/main/fs-error.ts

@@ -0,0 +1,56 @@
+// Maps low-level filesystem/OS error codes to a human-readable cause so that a
+// generic "write failed" or "timeout" can be reported as the specific root cause
+// (disk full, permission denied, ...). Pure + side-effect-free for testing.
+
+const DISK_ERROR_REASONS: Record<string, string> = {
+  ENOSPC: "Festplatte voll (ENOSPC)",
+  EDQUOT: "Speicher-Kontingent erschöpft (EDQUOT)",
+  EROFS: "Laufwerk schreibgeschützt (EROFS)",
+  EACCES: "Zugriff verweigert (EACCES)",
+  EPERM: "Operation nicht erlaubt (EPERM)",
+  EMFILE: "Zu viele offene Dateien (EMFILE)",
+  ENFILE: "System-Limit offener Dateien erreicht (ENFILE)",
+  EBUSY: "Datei/Laufwerk belegt (EBUSY)",
+  ENODEV: "Gerät nicht vorhanden (ENODEV)",
+  ENXIO: "Gerät getrennt (ENXIO)",
+  EIO: "Ein-/Ausgabefehler des Datenträgers (EIO)"
+};
+
+export function classifyDiskError(err: unknown): string | null {
+  const code = extractErrorCode(err);
+  if (code && DISK_ERROR_REASONS[code]) {
+    return DISK_ERROR_REASONS[code];
+  }
+  // Some errors arrive as plain strings/messages without a `.code`; fall back to
+  // scanning the text for a known code token.
+  const text = errorText(err);
+  for (const knownCode of Object.keys(DISK_ERROR_REASONS)) {
+    if (text.includes(knownCode)) {
+      return DISK_ERROR_REASONS[knownCode];
+    }
+  }
+  return null;
+}
+
+function extractErrorCode(err: unknown): string {
+  if (err && typeof err === "object") {
+    const code = (err as { code?: unknown }).code;
+    if (typeof code === "string") {
+      return code.toUpperCase();
+    }
+  }
+  return "";
+}
+
+function errorText(err: unknown): string {
+  if (typeof err === "string") {
+    return err;
+  }
+  if (err && typeof err === "object") {
+    const message = (err as { message?: unknown }).message;
+    if (typeof message === "string") {
+      return message;
+    }
+  }
+  return String(err ?? "");
+}