user: fix text selection on wrapped lines

scroll_pane: screen_to_item() now properly accounts for wrapped
lines using textwrap to compute actual character positions instead
of just using mouse_x directly.

selectable: new module with PUA markers for wrap-aware selection.
Not yet integrated into chat.rs but ready for future use. Uses
continuation markers to track logical vs visual lines.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/scroll_pane.rs

@@ -106,7 +106,27 @@ impl ScrollPaneState {
             let h = self.heights.get(line_idx).copied().unwrap_or(1) as i32;
             if (mouse_y as i32) < row + h {
                 let line_text: String = lines[line_idx].spans.iter().map(|s| s.content.as_ref()).collect();
-                let col = (mouse_x as usize).min(line_text.len());
+
+                // Which visual row within this wrapped line?
+                let visual_row_in_item = ((mouse_y as i32) - row).max(0) as usize;
+
+                // Use textwrap to find actual break positions
+                let wrap_width = self.cached_width as usize;
+                let wrapped = textwrap::wrap(&line_text, wrap_width);
+
+                // Sum lengths of previous wrapped rows to get char offset base
+                let char_base: usize = wrapped.iter()
+                    .take(visual_row_in_item)
+                    .map(|s| s.len())
+                    .sum();
+
+                // Add mouse x position within current row
+                let current_row_len = wrapped.get(visual_row_in_item)
+                    .map(|s| s.len())
+                    .unwrap_or(0);
+                let col = char_base + (mouse_x as usize).min(current_row_len);
+                let col = col.min(line_text.len());
+
                 return Some((line_idx, col));
             }
             row += h;