feat(documents): hide timeline density widget below lg (#385)

Tablet (640–1024px) is exactly the iPad audience for transcribers.
At 240 monthly bars on an 800px column the bars fall to ~3.3px wide,
well below the 44×44 touch-target floor. Bumps the visibility class
from hidden sm:block to hidden lg:block and matches the page.ts
matchMedia gate to (min-width: 1024px). Closes Leonie's tablet
touch-target finding.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/documents/+page.svelte

@@ -241,7 +241,7 @@ $effect(() => {
 		onblur={() => (qFocused = false)}
 	/>
 
-	<div class="mt-3 mb-4 hidden sm:block">
+	<div class="mt-3 mb-4 hidden lg:block">
 		<TimelineDensityFilter
 			density={data.density}
 			minDate={data.minDate}