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

lib/core/src/app_manager.cpp

@@ -52,6 +52,21 @@ void AppManager::endModal() {
 }
 
 void AppManager::handleKey(const KeyEvent& event) {
+    if (help_.isOpen()) {  // scrolls or closes; nothing reaches the App, Home included
+        help_.onKey(event);
+        redraw_ = true;
+        return;
+    }
+    if (event.key == Key::Help) {
+        std::vector<KeyHelp> rows;
+        foreground_->help(rows);
+        help::everywhere(rows);
+        const char* scope = foreground_->helpTitle();
+        const char* app = foregroundTitle();
+        help_.open(scope ? scope : app ? app : "Launcher", std::move(rows));
+        redraw_ = true;
+        return;
+    }
     if (modal_) {
         foreground_->onKey(event);
         return;
@@ -77,6 +92,7 @@ bool AppManager::takeRedraw() {
 }
 
 void AppManager::switchTo(App& app) {
+    help_.close();  // an App opened from elsewhere (a Notification, a command): its keys, not the last one's
     if (&app == foreground_) return;
     foreground_->onExit();
     foreground_ = &app;