Fix: allow selecting .tar.gz archives in macOS Finder by broadening input accept attribute and adding client-side validation

public/index.php

@@ -1533,7 +1533,7 @@ $currentUser = Auth::getUsername();
                             </div>
                         </div>
                         <div class="flex items-center gap-2.5 flex-wrap">
-                            <input type="file" id="backup-upload-input" accept=".tar.gz,.tgz" class="hidden" onchange="handleBackupUpload(this)">
+                            <input type="file" id="backup-upload-input" accept=".tar.gz,.tgz,.gz,.tar,application/gzip,application/x-gzip,application/x-tar,application/x-compressed-tar,application/octet-stream" class="hidden" onchange="handleBackupUpload(this)">
                             <button type="button" onclick="triggerBackupUpload()"
                                 class="inline-flex items-center gap-2 px-3.5 py-2.5 rounded-xl bg-anthracite-900 hover:bg-anthracite-750 border border-anthracite-600 text-xs font-semibold text-slate-200 hover:text-white transition shadow-sm">
                                 <svg class="w-4 h-4 text-purple-400" fill="none" stroke="currentColor" viewBox="0 0 24 24">