fix(frontend): lock document page to viewport with position: fixed

The global layout wraps pages in min-h-screen + main.py-6, which pushed
the h-screen document container below the sticky nav and caused page-level
scrolling. Switching to fixed inset-0 z-50 fully escapes the layout flow:

- DocumentTopBar always visible (no scrolling it away)
- PDF controls always visible
- Only the PDF canvas area scrolls
- DocumentBottomPanel moved inside the fixed container (logically grouped)

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -126,7 +126,7 @@ $effect(() => {
 	<title>{doc.title || doc.originalFilename || 'Dokument'}</title>
 </svelte:head>
 
-<div class="flex h-screen flex-col overflow-hidden bg-white" data-hydrated>
+<div class="fixed inset-0 z-50 flex flex-col overflow-hidden bg-white" data-hydrated>
 	<DocumentTopBar
 		doc={doc}
 		canWrite={data.canWrite ?? false}
@@ -149,7 +149,6 @@ $effect(() => {
 			}}
 		/>
 	</div>
-</div>
 
 	<DocumentBottomPanel
 		doc={doc}
@@ -163,3 +162,4 @@ $effect(() => {
 		activeAnnotationId={activeAnnotationId}
 		activeAnnotationPage={activeAnnotationPage}
 	/>
+</div>