feat(documents): autosize item descriptions (minRows 2, maxRows 16)

The 'Podrobny popis' textareas in the shared items editor and the invoice
page now grow with content like the rich-text editor (fixed rows={2} ->
minRows/maxRows autosize), capped at 16 rows; the manual vertical resize
handle stays.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/components/document/DocumentItemsEditor.tsx

@@ -214,7 +214,8 @@ function SortableItemRow({
           placeholder="Volitelný"
           InputProps={{ readOnly }}
           multiline
-          rows={2}
+          minRows={2}
+          maxRows={16}
           slotProps={{ htmlInput: { maxLength: itemDescriptionMaxLength } }}
           sx={{ "& textarea": { resize: "vertical" } }}
         />
@@ -352,7 +353,8 @@ function SortableItemRow({
             placeholder="Podrobný popis (volitelný)"
             InputProps={{ readOnly }}
             multiline
-            rows={2}
+            minRows={2}
+            maxRows={16}
             slotProps={{ htmlInput: { maxLength: itemDescriptionMaxLength } }}
             sx={{
               "& textarea": {

src/admin/pages/InvoiceDetail.tsx

@@ -317,7 +317,8 @@ function SortableInvoiceRow({
           onChange={(e) => onUpdate(index, "item_description", e.target.value)}
           placeholder="Volitelný"
           multiline
-          rows={2}
+          minRows={2}
+          maxRows={16}
           sx={{ "& textarea": { resize: "vertical" } }}
         />
         <Box
@@ -418,7 +419,8 @@ function SortableInvoiceRow({
             }
             placeholder="Podrobný popis (volitelný)"
             multiline
-            rows={2}
+            minRows={2}
+            maxRows={16}
             sx={{
               "& textarea": {
                 fontSize: "0.8rem",