Help: Fn+h lists the keys of the screen you're on, and no screen names its keys any more (#69)

Fn+h on any screen, text fields included, and ? outside Text Entry, open a
panel over the content area: the screen's own keys, then the ones that work
everywhere. Every App declares its keys for the state it is in (pages,
viewers, dialogs and text fields answer for themselves); the App manager
opens the panel and takes every key while it is open.

About 30 hint lines are gone, from every App. What stays on a screen is
state. The first-start Setup keeps its hints and teaches the key; a device
set up before gets one Toast, once. The guide and the FAQ open with it.
`key help` over the consoles.

476 host tests (8 new). Checked on the device with key help and screenshots:
the Launcher, all nine Apps and several of their states. 8.5 KB of flash and
40 bytes of static RAM. Decisions Q196 to Q203 in docs/milestones/U1.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/ui/widgets.h

@@ -6,6 +6,7 @@
 
 #include "canvas.h"
 #include "dialog_model.h"
+#include "key_help.h"
 #include "line_editor.h"
 #include "list_model.h"
 #include "text_wrap.h"
@@ -59,6 +60,9 @@ void lineEditor(Canvas& c, const LineEditor& editor, theme::Rect area);
 
 void dialog(Canvas& c, const std::string& title, const std::string& message, const DialogModel& model);
 
+// The help panel (Fn+h): the keys of the screen under it, over the whole content area.
+void help(Canvas& c, const HelpModel& model);
+
 void toast(Canvas& c, const Toast& t);
 
 }  // namespace widgets