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/DocumentBottomPanel.svelte

@@ -73,11 +73,16 @@ let isDragging = $state(false);
 let dragStartY = 0;
 let dragStartHeight = 0;
 
+function fullHeight() {
+	const topbar = document.querySelector('[data-topbar]');
+	return window.innerHeight - (topbar?.getBoundingClientRect().height ?? 0);
+}
+
 function openTab(tab: Tab) {
 	activeTab = tab;
 	if (!open) {
 		open = true;
-		if (height <= MIN_HEIGHT) height = Math.floor(window.innerHeight * 0.8);
+		if (height <= MIN_HEIGHT) height = fullHeight();
 	}
 }
 
@@ -96,7 +101,7 @@ function onDragMove(e: PointerEvent) {
 	if (!isDragging) return;
 	const delta = dragStartY - e.clientY; // positive = dragging up = bigger panel
 	const newHeight = dragStartHeight + delta;
-	const maxHeight = Math.floor(window.innerHeight * 0.8);
+	const maxHeight = fullHeight();
 
 	if (newHeight <= MIN_HEIGHT + 20) {
 		// collapsed past threshold → close