Account-rotation logging + transient cooldown fixes

- New dedicated account-rotation.log (audit-style) so multi-account/key
  rotation flow is visible without rd_downloader.log noise
- Mega-Debrid: always show "Account X/Y (masked@login)" label even with
  one account, and log a clear "TESTE Account fuer Link-Generierung..."
  line BEFORE every network call so the user sees which account is in
  play even if the call hangs
- Mega-Debrid: on FAILED, log which account will be tried next
  (skipping disabled/limited/cooldown ones), so the rotation is auditable
- Mega-Web "Antwort leer" / empty body now uses 20s cooldown instead of
  120s — empty responses are typically transient server hiccups, not
  real failures (caused healthy accounts to be unfairly blocked)
- Generic transport/unknown errors: 30s cooldown instead of 120s
- Global stall watchdog no longer counts items in "validating" status —
  per-item validating watchdog already handles them and gives the
  multi-account rotation enough time. Without this fix the global
  watchdog could abort the unrestrict mid-rotation (e.g. account 3 of 3
  still being tested) just because no download bytes had arrived
- Same logging treatment applied to Debrid-Link key rotation

src/main/download-manager.ts

@@ -7660,6 +7660,15 @@ export class DownloadManager extends EventEmitter {
       return;
     }
 
+    // Only items that are actually DOWNLOADING (have an open HTTP body that
+    // should be making progress) count toward the global stall watchdog.
+    // Items in "validating" are still in the unrestrict phase — they're handled
+    // by the per-item validating watchdog above (VALIDATING_STUCK_MS) which
+    // gives the multi-account/multi-key rotation enough time. Without this
+    // exclusion the global watchdog would abort the unrestrict mid-rotation
+    // (e.g. account 3 of 3 still being tested) just because no bytes had been
+    // downloaded recently — which is correct, since unrestrict doesn't
+    // produce download bytes.
     let stalledCount = 0;
     let diskBlockedCount = 0;
     for (const active of this.activeTasks.values()) {
@@ -7671,7 +7680,7 @@ export class DownloadManager extends EventEmitter {
         continue;
       }
       const item = this.session.items[active.itemId];
-      if (item && (item.status === "downloading" || item.status === "validating")) {
+      if (item && item.status === "downloading") {
         stalledCount += 1;
       }
     }
@@ -7689,7 +7698,7 @@ export class DownloadManager extends EventEmitter {
         continue;
       }
       const item = this.session.items[active.itemId];
-      if (item && (item.status === "downloading" || item.status === "validating")) {
+      if (item && item.status === "downloading") {
         active.abortReason = "stall";
         active.abortController.abort("stall");
       }