fix(frontend): open bottom panel to full height below the document header

Instead of an arbitrary 80 % cap, the panel now measures the actual
DocumentTopBar height at open time and fills the remaining viewport
exactly — so the PDF is fully covered and the drawer reaches right up
to the header. Drag-to-shrink still works as before.

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

frontend/src/lib/components/DocumentTopBar.svelte

@@ -59,6 +59,7 @@ const compactMeta = $derived.by(() => {
 
 <div
 	class="z-20 flex shrink-0 items-center justify-between border-b border-brand-sand bg-white px-6 py-3 shadow-sm"
+	data-topbar
 >
 	<!-- Left: back + title -->
 	<div class="flex min-w-0 items-center gap-4 overflow-hidden">