fix(pdf): make isLoaded reactive so nav buttons are enabled after load

pdfDoc was a plain variable (not \$state), so renderer.isLoaded had no
reactive dependencies in Svelte 5. PdfControls received isLoaded=false
permanently, keeping the next-page button disabled while zoom buttons
(which have no disabled attribute) still worked.

Fix: derive isLoaded from totalPages (\$state) via totalPages > 0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/hooks/usePdfRenderer.svelte.ts

@@ -183,7 +183,7 @@ export function createPdfRenderer() {
 			return error;
 		},
 		get isLoaded() {
-			return pdfDoc !== null;
+			return totalPages > 0;
 		},
 		get pdfjsReady() {
 			return pdfjsReady;