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/lib/document/timeline.ts

@@ -193,10 +193,10 @@ export function buildDensityUrl(filters: DensityFilters = {}): string {
 }
 
 /**
- * Loads the density data for the timeline widget. Mobile (sm: breakpoint and below)
- * and calendar view both skip the request entirely — the widget isn't rendered
- * there. A non-ok response or network failure degrades to an empty bucket list
- * instead of throwing, so the document list page keeps rendering.
+ * Loads the density data for the timeline widget. Tablet and below (lg breakpoint,
+ * <1024px) and calendar view both skip the request entirely — the widget isn't
+ * rendered there. A non-ok response or network failure degrades to an empty
+ * bucket list instead of throwing, so the document list page keeps rendering.
  */
 export async function fetchDensity(
 	fetch: typeof globalThis.fetch,