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/apps/demo_app.cpp

@@ -25,6 +25,15 @@ void DemoApp::notify(const char* text, NotificationLevel level) {
     bus_.publish(Event::withText(EventType::Notification, text, static_cast<int32_t>(level)));
 }
 
+void DemoApp::help(std::vector<KeyHelp>& out) const {
+    if (dialog_) return help::dialog(out);
+    switch (page_) {
+        case Page::Menu: help::list(out, "try the widget"); break;
+        case Page::Text: out.push_back({"; .", "scroll"}); break;
+        case Page::Editor: help::textEntry(out, "show the text as a Toast"); break;
+    }
+}
+
 bool DemoApp::onKey(const KeyEvent& e) {
     requestRedraw();
     if (dialog_) {
@@ -97,7 +106,7 @@ void DemoApp::draw(Canvas& c) {
         }
         case Page::Editor:
             c.setTextColor(theme::kMuted);
-            c.drawString("Type (opt ' e = \xC3\xA9). Enter: toast", 4, area.y + 4);
+            c.drawString("A line editor", 4, area.y + 4);
             widgets::lineEditor(c, editor_, {4, area.y + 22, area.w - 8, 0});
             c.setTextColor(theme::kMuted);
             c.drawString((std::to_string(editor_.text().size()) + "/39 bytes").c_str(), 4, area.y + 44);