fix: remove EnableMouseCapture to allow native terminal text selection

Root cause: EnableMouseCapture sent ?1000h/?1002h/?1006h/?1015h to the
terminal, redirecting ALL mouse events (including drag) to the app.
This blocked the terminal's native text selection.

Changes:
- terminal.rs: remove EnableMouseCapture, update shutdown comment.
  DECRST sequences are idempotent defensive cleanup so left in place.
- event.rs: remove ScrollUp, ScrollDown, MouseDown, MouseDrag,
  MouseUp variants from UserEvent enum.
- ui/mod.rs: remove background mouse event reader thread, all
  mouse/scroll event handlers in main loop and permission alert
  loop, selection-copy handler (y key), selection-clear handler
  (Esc key), and Esc double-tap guard that checked selection_active.
- slash.rs: update help text — 'drag + Cmd/Ctrl+C' replaces mouse
  selection and mouse scroll entries.

Users now use native terminal text selection (drag to select,
Cmd+C/Ctrl+Shift+C to copy, PgUp/PgDn to scroll).

Author: Yogthos

src/event.rs

@@ -157,20 +157,6 @@ pub enum AgentEvent {
 #[derive(Debug, Clone)]
 pub enum UserEvent {
     Key(crossterm::event::KeyEvent),
-    ScrollUp,
-    ScrollDown,
-    MouseDown {
-        row: u16,
-        col: u16,
-    },
-    MouseDrag {
-        row: u16,
-        col: u16,
-    },
-    MouseUp {
-        row: u16,
-        col: u16,
-    },
     Paste(String),
     /// Terminal was resized. Carries no payload — the renderer queries
     /// `crossterm::terminal::size()` directly; the variant is just a kick