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/chat.rs

@@ -11,7 +11,7 @@ use ratatui::{
     Frame,
 };
 
-use super::{ActivePane, App, Marker, PaneState, SCREEN_LEGEND};
+use super::{ActivePane, App, Marker, PaneState, screen_legend};
 
 impl App {
     /// Draw the main (F1) screen — four-pane layout with status bar.
@@ -62,7 +62,7 @@ impl App {
         // Draw autonomous pane
         let auto_active = self.active_pane == ActivePane::Autonomous;
         draw_pane(frame, auto_area, "autonomous", &mut self.autonomous, auto_active,
-            Some(SCREEN_LEGEND));
+            Some(&screen_legend()));
 
         // Draw tools pane
         let tools_active = self.active_pane == ActivePane::Tools;