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/logger.ts

@@ -1,7 +1,24 @@
 import fs from "node:fs";
 import { logTimestamp } from "./log-timestamp";
+import { recordRecentError } from "./error-ring";
 import path from "node:path";
 
+export function isDebugFlagEnabled(value: string | undefined): boolean {
+  if (!value) {
+    return false;
+  }
+  return /^(1|true|yes|on)$/i.test(value.trim());
+}
+
+// Read once at startup. Enabling verbose DEBUG logging on the (unattended) server
+// is a deliberate support action that requires a restart — the runtime-toggleable
+// channel is the trace log, not this.
+const DEBUG_ENABLED = isDebugFlagEnabled(process.env.RD_DEBUG);
+
+export function isDebugLoggingEnabled(): boolean {
+  return DEBUG_ENABLED;
+}
+
 let logFilePath = path.resolve(process.cwd(), "rd_downloader.log");
 let fallbackLogFilePath: string | null = null;
 const LOG_FLUSH_INTERVAL_MS = 120;
@@ -204,12 +221,19 @@ function ensureExitHook(): void {
   process.once("exit", flushSyncPending);
 }
 
-function write(level: "INFO" | "WARN" | "ERROR", message: string): void {
+function write(level: "DEBUG" | "INFO" | "WARN" | "ERROR", message: string): void {
   ensureExitHook();
-  const line = `${logTimestamp()} [${level}] ${message}\n`;
+  const ts = logTimestamp();
+  const line = `${ts} [${level}] ${message}\n`;
   pendingLines.push(line);
   pendingChars += line.length;
 
+  // Single chokepoint: every WARN/ERROR also lands in the in-memory ring so
+  // "what failed recently" is answerable even after the file rotates.
+  if (level === "ERROR" || level === "WARN") {
+    recordRecentError(level, message, ts);
+  }
+
   for (const listener of logListeners) {
     try { listener(line); } catch {  }
   }
@@ -230,6 +254,9 @@ function write(level: "INFO" | "WARN" | "ERROR", message: string): void {
 }
 
 export const logger = {
+  // Gated to a no-op when RD_DEBUG is unset so verbose call sites cost nothing
+  // (no formatting, no allocation) in the normal/production path.
+  debug: DEBUG_ENABLED ? (msg: string): void => write("DEBUG", msg) : (_msg: string): void => {},
   info: (msg: string): void => write("INFO", msg),
   warn: (msg: string): void => write("WARN", msg),
   error: (msg: string): void => write("ERROR", msg)