Screenshots: Fn+p on every screen (#83)

Fn+p saves the screen as it is to /screenshots, as the Shell's `screenshot`
does, from anywhere: text fields, dialogs and the help panel included. The
key never reaches an App.

It refuses on Settings > Debug Console, which shows the token: a picture of
that page is a copy of the token in a file (App::showsSecret). `key shot`
presses it over the consoles.

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

src/apps/settings_app.h

@@ -52,6 +52,7 @@ class SettingsApp : public App {
     void draw(Canvas& c) override;
     void help(std::vector<KeyHelp>& out) const override;
     const char* helpTitle() const override;
+    bool showsSecret() const override { return page_ == Page::Debug; }  // the Debug Console's token
 
    private:
     enum class Page { Menu, Text, Choice, About, Wifi, Firmware, Debug };

src/apps/shell_app.cpp

@@ -26,7 +26,7 @@ void ShellApp::onEnter() {
     scroll_ = 0;
     confirm_.reset();
     // What Tab completes besides the firmware's own commands, written as `help` writes them.
-    ownHelp_ = "help | clear | quit | exit\nkey up|down|left|right|select|back|home|del|tab|space|help\n";
+    ownHelp_ = "help | clear | quit | exit\nkey up|down|left|right|select|back|home|del|tab|space|help|shot\n";
     log_.clear();
     log_.add(open_ ? "The console's commands. `help` lists them." : "No memory for the Shell: leave an App, or stop IRC.");
 }