user: ScreenView trait, overlay screens extracted from App

Convert F2-F5 screens to ScreenView trait with tick() method.
Each screen owns its view state (scroll, selection, expanded).
State persists across screen switches.

- ThalamusScreen: owns sampling_selected, scroll
- ConsciousScreen: owns scroll, selected, expanded
- SubconsciousScreen: owns selected, log_view, scroll
- UnconsciousScreen: owns scroll

Removed from App: Screen enum, debug_scroll, debug_selected,
debug_expanded, agent_selected, agent_log_view, sampling_selected,
set_screen(), per-screen key handling, draw dispatch.

App now only draws the interact (F1) screen. Overlay screens are
drawn by the event loop via ScreenView::tick. F-key routing and
screen instantiation to be wired in event_loop next.

InteractScreen (state-driven, reading from agent entries) is the
next step — will eliminate the input display race condition.

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

src/user/event_loop.rs

@@ -409,7 +409,7 @@ pub async fn run(
                         if key.kind != KeyEventKind::Press { continue; }
                         app.handle_key(key);
                         idle_state.user_activity();
-                        if app.screen == tui::Screen::Thalamus {
+                        if false { // TODO: check active screen is thalamus
                             let tx = channel_tx.clone();
                             tokio::spawn(async move {
                                 let result = crate::thalamus::channels::fetch_all_channels().await;