unify tool tracking: ActiveToolCall with JoinHandle

One data structure for all in-flight tool calls — metadata for
TUI display + JoinHandle for result collection and cancellation.
Agent spawns tool calls via tokio::spawn, pushes to shared
Arc<Mutex<Vec<ActiveToolCall>>>. TUI reads metadata, can abort().
No separate inflight/background collections.

Non-background: awaited after stream ends.
Background: persists, drained at next turn start.

Co-Developed-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/tui/main_screen.rs

@@ -17,7 +17,7 @@ impl App {
     /// Draw the main (F1) screen — four-pane layout with status bar.
     pub(crate) fn draw_main(&mut self, frame: &mut Frame, size: Rect) {
         // Main layout: content area + active tools overlay + status bar
-        let active_tools = self.active_tools.read().unwrap();
+        let active_tools = self.active_tools.lock().unwrap();
         let tool_lines = active_tools.len() as u16;
         let main_chunks = Layout::default()
             .direction(Direction::Vertical)