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/error-ring.ts

@@ -0,0 +1,45 @@
+export interface ErrorRingEntry {
+  ts: string;
+  level: string;
+  message: string;
+}
+
+export interface ErrorRing {
+  push: (entry: ErrorRingEntry) => void;
+  snapshot: () => ErrorRingEntry[];
+  clear: () => void;
+  size: () => number;
+}
+
+export function createErrorRing(capacity: number): ErrorRing {
+  const limit = Math.max(1, Math.floor(capacity));
+  const buffer: ErrorRingEntry[] = [];
+  return {
+    push(entry: ErrorRingEntry): void {
+      buffer.push(entry);
+      while (buffer.length > limit) {
+        buffer.shift();
+      }
+    },
+    snapshot(): ErrorRingEntry[] {
+      return buffer.slice();
+    },
+    clear(): void {
+      buffer.length = 0;
+    },
+    size(): number {
+      return buffer.length;
+    }
+  };
+}
+
+const RECENT_ERROR_CAPACITY = 200;
+const recentErrors = createErrorRing(RECENT_ERROR_CAPACITY);
+
+export function recordRecentError(level: string, message: string, ts: string): void {
+  recentErrors.push({ level, message, ts });
+}
+
+export function getRecentErrors(): ErrorRingEntry[] {
+  return recentErrors.snapshot();
+}