input: Fix TextInput cursor getting stuck on Down key press (#526)

## Before

![2025-01-03 13 55 08
before](https://github.com/user-attachments/assets/3714e316-289b-4426-b18a-23ac718c05e6)

## After

![2025-01-03 13 54 22
after](https://github.com/user-attachments/assets/5d82a864-6cda-4191-adee-26634ef93506)

crates/ui/src/input/input.rs

@@ -365,7 +365,7 @@ impl TextInput {
         let index_res = target_line.index_for_position(approx_pos, line_height);
 
         let new_local_index = match index_res {
-            Ok(i) => i,
+            Ok(i) => i + 1,
             Err(i) => i,
         };