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/table_handler_tests.rs

@@ -606,17 +606,18 @@ mod tests {
       .take(100)
       .collect();
       app.data.radarr_data.movies.set_items(movies_vec);
+      app.data.radarr_data.movies.state.select(Some(50));
       TableHandlerUnit::new(key, &mut app, ActiveRadarrBlock::Movies, None).handle();
 
       if key == Key::PgUp {
         assert_str_eq!(
           app.data.radarr_data.movies.current_selection().title.text,
-          "Test 79"
+          "Test 30"
         );
       } else {
         assert_str_eq!(
           app.data.radarr_data.movies.current_selection().title.text,
-          "Test 20"
+          "Test 70"
         );
       }
     }