fix: page up and down stop at the table edges instead of wrapping

A page is 20 rows and the index wrapped modulo one less than the row
count, so the last row was unreachable by paging, 21-row and 5-row tables
did nothing on page down from the first row, and 4-row tables cycled
0, 2, 1 and never reached row 3. Page down now clamps to the last row and
page up clamps to the first.

With no division left, a single-row table is safe, so the early return
only guards the empty table again, matching scroll_to_top and
scroll_to_bottom.

src/handlers/handler_proptest.rs

@@ -171,10 +171,19 @@ mod property_tests {
       table.set_items(movies);
 
       for i in 0..page_ops {
+        let previous_index = table.state.selected().unwrap();
+
         if i % 2 == 0 {
           table.page_down();
+
+          prop_assert_eq!(
+            table.state.selected(),
+            Some((previous_index + 20).min(list_size - 1))
+          );
         } else {
           table.page_up();
+
+          prop_assert_eq!(table.state.selected(), Some(previous_index.saturating_sub(20)));
         }
 
         let current = table.current_selection();