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

lib/core/src/app.h

@@ -39,6 +39,10 @@ class App {
 
     virtual void draw(Canvas& canvas) = 0;
 
+    // True while the screen shows something a picture of it shouldn't hold: the screenshot key
+    // (Fn+p, issue #83) then refuses, and says so.
+    virtual bool showsSecret() const { return false; }
+
     // An App whose screen is costly to draw again (a picture decoded from the card, issue #45) can
     // keep what it drew: while this is true its part of the screen isn't cleared before draw(),
     // which then draws only what changed. contentLost() says that it was cleared after all, or

lib/core/src/app_keys.h

@@ -27,6 +27,7 @@ inline constexpr KeyHelp kEverywhere[] = {
     {"Fn `", "home, the Launcher"},
     {"; . , /", "arrows (Fn+ while typing)"},
     {"Fn h  ?", "these keys (? not typing)"},
+    {"Fn p", "a screenshot, on the card"},
 };
 
 // dialog: A question

lib/core/src/key_event.h

@@ -17,6 +17,7 @@ enum class Key : uint8_t {
     Tab,
     Delete,
     Help,  // Fn+h anywhere, or ? outside Text Entry: the keys of this screen (issue #69)
+    Screenshot,  // Fn+p anywhere: the screen as a PNG on the card (issue #83). Never reaches an App
 };
 
 struct KeyEvent {

lib/input/src/key_mapper.cpp

@@ -106,6 +106,13 @@ void KeyMapper::onChar(char c, const RawKeys& keys, std::vector<KeyEvent>& out)
                     return;
                 }
                 break;
+            case 'p':
+            case 'P':
+                if (keys.fn) {  // Fn+p: a screenshot, while typing too
+                    out.push_back(KeyEvent::of(Key::Screenshot));
+                    return;
+                }
+                break;
             case '?':
                 if (!textEntry_ && !keys.fn) {  // ? alone, when it wouldn't be typed
                     out.push_back(KeyEvent::of(Key::Help));