feat(export): add branding footer to all 5 export formats and PDF spinner

- Add "Generated with ResolutionFlow — https://resolutionflow.com" footer
  to markdown, text, HTML, PSA formats (both troubleshooting and procedural
  variants — 8 generators total)
- Fix PDF @page CSS: "Powered by ResolutionFlow" now appears on every PDF,
  not just for users with a custom logo (removed the has_custom_logo conditional)
- Add Loader2 spinner icon to PDF download button in ExportPreviewModal
  when pdfLoading is true, replacing the static Download icon

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/session/ExportPreviewModal.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from 'react'
-import { Copy, Download, Check, RotateCcw, FileDown } from 'lucide-react'
+import { Copy, Download, Check, RotateCcw, FileDown, Loader2 } from 'lucide-react'
 import { Modal } from '@/components/common/Modal'
 import { cn } from '@/lib/utils'
 import type { RedactionSummary } from '@/types'
@@ -149,7 +149,11 @@ export function ExportPreviewModal({
               'disabled:opacity-60 disabled:cursor-not-allowed'
             )}
           >
-            <Download className="h-4 w-4" />
+            {pdfLoading ? (
+              <Loader2 className="h-4 w-4 animate-spin" />
+            ) : (
+              <Download className="h-4 w-4" />
+            )}
             {pdfLoading ? 'Generating PDF...' : 'Download PDF'}
           </button>
         </div>