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.h

@@ -37,6 +37,9 @@ class AppManager {
     App& foreground() const { return *foreground_; }
     const char* foregroundTitle() const;  // nullptr for the Launcher
 
+    // The help panel (Fn+h): open, it takes every key, and the App sees none of them.
+    const HelpModel& help() const { return help_; }
+
     // True once after the screen needs redrawing (App switch, or the App asked for it).
     bool takeRedraw();
 
@@ -47,6 +50,7 @@ class AppManager {
     App& launcher_;
     App* foreground_;
     std::vector<AppInfo> apps_;
+    HelpModel help_;
     bool redraw_ = true;
     bool modal_ = false;
 };