add comment on App

src/tui.zig

@@ -192,6 +192,16 @@ pub const OptionsRow = struct {
     contract: ?zfin.OptionContract = null,
 };
 
+/// Root widget for the interactive TUI. Implements the vaxis `vxfw.Widget`
+/// interface via `widget()`, which wires `typeErasedEventHandler` and
+/// `typeErasedDrawFn` as callbacks. Passed to `vxfw.App.run()` as the
+/// top-level widget; vaxis drives the event loop, calling back into App
+/// for key/mouse/init events and for each frame's draw.
+///
+/// Owns all application state: the active tab, cached data for each tab,
+/// navigation/scroll positions, input mode, and a reference to the
+/// `DataService` for fetching financial data. Tab-specific rendering and
+/// data loading are delegated to the `tui/*_tab.zig` modules.
 pub const App = struct {
     allocator: std.mem.Allocator,
     config: zfin.Config,