Add widget kit, input layer with Compose Key, Launcher and demo apps

Steps 5 and 6 of M0.
- lib/ui (host-tested): ListModel, LineEditor (UTF-8, byte-limited),
  wrapText, ToastQueue, DialogModel
- lib/input (host-tested): KeyMapper turns keyboard state into logical
  keys (Fn arrows, ` Back, Fn+` Home) and composes accents with opt
- src/ui: off-screen 16-bit frame pushed in one transfer (fixes the
  flicker), Status Bar, list, text, line editor, dialog, Toast widgets,
  Notifier (Toast + beep + LED), vendored public-domain Latin-1 fonts
- Apps: minimal Launcher, Widget demo, Diagnostics (SD erase now
  behind a confirmation dialog)

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

src/ui/screen.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "app_manager.h"
+#include "canvas.h"
+#include "widgets.h"
+
+namespace roro {
+
+// Composes the frame (Status Bar, foreground App, Toast) off-screen and pushes it to the display.
+class Screen {
+   public:
+    bool begin();  // allocates the frame buffer (~64 KB)
+    void render(AppManager& apps, const StatusInfo& status, const Toast* toast);
+    Canvas& canvas() { return canvas_; }
+
+   private:
+    Canvas canvas_;
+};
+
+}  // namespace roro