fix: revert draw gating; replace frame-count scroll with wall-clock timer

- Revert src/main.rs to draw-at-top-of-loop (original structure) so
  auto-scrolling is no longer coupled to event rate. Lock is released
  before blocking on recv() so the network task is never starved.

- Replace ticks_until_scroll counter in on_ui_scroll_tick() with an
  Instant + Duration (default 100ms). Scroll now advances at a fixed
  wall-clock rate (10 chars/sec) regardless of draw rate or tick rate,
  which is the correct long-term fix for the animation regression.

- Update tests: assert scroll_interval == 100ms in default/new tests;
  rewrite test_on_ui_scroll_tick to cover zero-interval (always fires)
  and long-interval (suppressed) cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C3FpbniZme9mJqgFkMVowz

src/main.rs

@@ -276,12 +276,12 @@ async fn start_ui(
 
   let input_events = Events::new();
 
-  {
-    let mut app = app.lock().await;
-    terminal.draw(|f| ui(f, &mut app))?;
-  }
-
   loop {
+    {
+      let mut app = app.lock().await;
+      terminal.draw(|f| ui(f, &mut app))?;
+    }
+
     match input_events.next()? {
       Some(InputEvent::KeyEvent(key)) => {
         let mut app = app.lock().await;
@@ -291,13 +291,11 @@ async fn start_ui(
         }
 
         handlers::handle_events(key, &mut app);
-        terminal.draw(|f| ui(f, &mut app))?;
       }
 
       Some(InputEvent::Tick) => {
         let mut app = app.lock().await;
         app.on_tick().await;
-        terminal.draw(|f| ui(f, &mut app))?;
       }
       None => break,
     }