tui: fix cursor position to use character count, not byte count

self.cursor is a byte index into the string. When scanning the buffer,
we need to compare character positions, not byte positions or widths.

Convert self.cursor to a character count before comparing with the
buffer scan. Count each non-empty cell as 1 character (the buffer
already represents visual cells, so width doesn't matter here).

poc-agent/src/tui.rs

@@ -821,7 +821,11 @@ impl App {
         // Cursor position: scan the rendered buffer to find where the cursor should be.
         // This matches ratatui's actual word wrapping instead of trying to simulate it.
         let buffer = frame.buffer_mut();
-        let cursor_char = prompt.len() + self.cursor;  // Total chars from start (prompt + input)
+        
+        // Convert byte index to character index for the input portion
+        let input_chars_before_cursor = self.input[..self.cursor].chars().count();
+        let cursor_char_pos = prompt.chars().count() + input_chars_before_cursor;
+        
         let mut char_count = 0usize;
         let mut cursor_x = input_area.x;
         let mut cursor_y = input_area.y;
@@ -833,14 +837,13 @@ impl App {
                     let symbol = cell.symbol();
                     // Count visible characters (skip zero-width and empty)
                     if !symbol.is_empty() {
-                        let width = symbol.width();
-                        if char_count + width > cursor_char {
+                        if char_count == cursor_char_pos {
                             // Found the cursor position
                             cursor_x = x;
                             cursor_y = y;
                             break;
                         }
-                        char_count += width;
+                        char_count += 1;
                     }
                 }
             }