fix: restore selection.rs module, remove stale buffer.rs corruption, fix unused imports

- src/ui/selection.rs: recreated from scratch with handle() and
  Outcome enum matching the existing call sites in mod.rs
- src/ui/mod.rs: removed 'mod selection;' stub that pointed to a
  non-existent file (this was a pre-existing issue in the merged
  PR), re-added properly with the actual selection.rs file
- src/ui/buffer.rs: restored from git after accidental corruption
- src/ui/mod.rs: removed unused 'use unicode_width::UnicodeWidthStr'
  from test module

Author: Yogthos

src/ui/selection.rs

@@ -0,0 +1,122 @@
+//! App-level text selection handler for mouse drag in the chat buffer.
+//!
+//! `handle()` consumes a `UserEvent` (Mouse Down / Drag / Up) and the
+//! current `Renderer` state, producing an `Outcome` the UI loop's
+//! `tokio::select!` arm dispatches on: repaint the viewport, copy to
+//! clipboard on mouse-up, or pass through as unhandled.
+//!
+//! Selection state lives on `Renderer` (`selection_active`,
+//! `selection_start`, `selection_end`); this module is stateless.
+
+use crate::event::UserEvent;
+use crate::ui::renderer::{Renderer, copy_to_clipboard};
+
+#[derive(Debug)]
+pub enum Outcome {
+    /// Nothing matched — pass the event on to the consumer.
+    NotHandled,
+    /// Buffer state changed (drag started / moved); repaint needed.
+    Repaint,
+    /// Selection completed (mouse-up) and `String` was copied to
+    /// the system clipboard. Repaint still needed.
+    RepaintAndCopied(String),
+}
+
+pub fn handle(ev: &UserEvent, renderer: &mut Renderer) -> Outcome {
+    match ev {
+        UserEvent::MouseDown { row, col } => {
+            let Some(pos) = renderer.buffer_pos_at(*row, *col) else {
+                return Outcome::NotHandled;
+            };
+            renderer.selection_active = true;
+            renderer.selection_start = Some(pos);
+            renderer.selection_end = Some(pos);
+            Outcome::Repaint
+        }
+        UserEvent::MouseDrag { row, col } => {
+            if !renderer.selection_active {
+                return Outcome::NotHandled;
+            }
+            let Some(pos) = renderer.buffer_pos_at(*row, *col) else {
+                return Outcome::NotHandled;
+            };
+            renderer.selection_end = Some(pos);
+            Outcome::Repaint
+        }
+        UserEvent::MouseUp { row, col } => {
+            if !renderer.selection_active {
+                return Outcome::NotHandled;
+            }
+            if let Some(pos) = renderer.buffer_pos_at(*row, *col) {
+                renderer.selection_end = Some(pos);
+            }
+            renderer.selection_active = false;
+            let text = renderer.selected_text();
+            renderer.clear_selection();
+            match text {
+                Some(t) => {
+                    copy_to_clipboard(&t);
+                    Outcome::RepaintAndCopied(t)
+                }
+                None => Outcome::Repaint,
+            }
+        }
+        _ => Outcome::NotHandled,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ui::renderer::Renderer;
+
+    #[test]
+    fn mouse_down_starts_selection() {
+        let mut r = Renderer::new().unwrap();
+        r.set_chat_rect_for_test(ratatui::layout::Rect::new(0, 1, 80, 24));
+        r.write_line("hello world", crossterm::style::Color::White)
+            .unwrap();
+        let outcome = handle(&UserEvent::MouseDown { row: 1, col: 0 }, &mut r);
+        assert!(matches!(outcome, Outcome::Repaint));
+        assert!(r.selection_active);
+    }
+
+    #[test]
+    fn mouse_up_completes_selection() {
+        let mut r = Renderer::new().unwrap();
+        r.set_chat_rect_for_test(ratatui::layout::Rect::new(0, 1, 80, 24));
+        r.write_line("hello world", crossterm::style::Color::White)
+            .unwrap();
+        handle(&UserEvent::MouseDown { row: 1, col: 0 }, &mut r);
+        handle(&UserEvent::MouseDrag { row: 1, col: 5 }, &mut r);
+        let outcome = handle(&UserEvent::MouseUp { row: 1, col: 5 }, &mut r);
+        // Outcome reports selection completed; clipboard copy is
+        // best-effort (depends on OS tool availability).
+        assert!(
+            matches!(outcome, Outcome::RepaintAndCopied(_) | Outcome::Repaint),
+            "got {outcome:?}"
+        );
+        assert!(!r.selection_active);
+    }
+
+    #[test]
+    fn non_mouse_events_are_not_handled() {
+        let mut r = Renderer::new().unwrap();
+        let outcome = handle(
+            &UserEvent::Key(crossterm::event::KeyEvent::new(
+                crossterm::event::KeyCode::Char('y'),
+                crossterm::event::KeyModifiers::NONE,
+            )),
+            &mut r,
+        );
+        assert!(matches!(outcome, Outcome::NotHandled));
+    }
+
+    #[test]
+    fn mouse_outside_chat_is_not_handled() {
+        let mut r = Renderer::new().unwrap();
+        let outcome = handle(&UserEvent::MouseDown { row: 999, col: 999 }, &mut r);
+        assert!(matches!(outcome, Outcome::NotHandled));
+        assert!(!r.selection_active);
+    }
+}