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 {