fix(ui): let a truncating label actually truncate

A span marked both shrink-0 and truncate never gives up width, so a
long source name on a collapsed priority group squeezed the metric
label at phone width, and a long month marker in the document flow
could overrun its cell. Both shrink now and cut with an ellipsis.

src/components/documents/document-month-grid.tsx

@@ -76,7 +76,7 @@ function FlowMonthMarker({ label }: { label: string }) {
       data-slot="document-flow-month"
       className="absolute inset-x-0 -top-6 flex h-6 items-center gap-2"
     >
-      <span className="text-muted-foreground shrink-0 truncate text-xs font-medium tracking-wide uppercase">
+      <span className="text-muted-foreground min-w-0 truncate text-xs font-medium tracking-wide uppercase">
         {label}
       </span>
       <span aria-hidden className="bg-border h-px flex-1" />