fix(design): keep sortable table headers uppercase (#1558)

The sortable headers on /bookkeeping and /invoices wrap their label in a
button, and preflight's text-transform: none on buttons swallowed the
uppercase from TH_CLASS, so sorted lists showed sentence-case heads next
to every other table's uppercase idiom. Re-apply uppercase on the sort
control itself.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/(dashboard)/invoices/page.tsx

@@ -168,10 +168,12 @@ function SortableHeader({
       className={cn(TH_CLASS, className)}
       aria-sort={direction === 'asc' ? 'ascending' : direction === 'desc' ? 'descending' : 'none'}
     >
+      {/* Preflight sets text-transform: none on buttons, which would drop the
+          TH_CLASS uppercase idiom inside the sort control. */}
       <button
         type="button"
         className={cn(
-          '-mx-2 inline-flex min-h-10 items-center gap-1 rounded-sm px-2 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2',
+          '-mx-2 inline-flex min-h-10 items-center gap-1 rounded-sm px-2 uppercase focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2',
           align === 'right' && 'ml-auto justify-end',
         )}
         aria-label={sortLabel}

components/bookkeeping/JournalEntryList.tsx

@@ -150,10 +150,12 @@ function SortableHeader({
       className={cn(TH_CLASS, className)}
       aria-sort={active ? (direction === 'asc' ? 'ascending' : 'descending') : 'none'}
     >
+      {/* Preflight sets text-transform: none on buttons, which would drop the
+          TH_CLASS uppercase idiom inside the sort control. */}
       <button
         type="button"
         className={cn(
-          '-mx-2 inline-flex min-h-10 items-center gap-1 rounded-sm px-2 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2',
+          '-mx-2 inline-flex min-h-10 items-center gap-1 rounded-sm px-2 uppercase focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2',
           align === 'right' && 'ml-auto justify-end',
         )}
         aria-label={sortLabel}