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/apps/demo_app.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "app.h"
+#include "dialog_model.h"
+#include "event_bus.h"
+#include "line_editor.h"
+#include "list_model.h"
+#include "ui/theme.h"
+
+namespace roro {
+
+// Exercises every widget: text view, line editor, dialog and Toasts.
+class DemoApp : public App {
+   public:
+    explicit DemoApp(EventBus& bus) : bus_(bus) {}
+    void onEnter() override;
+    bool onKey(const KeyEvent& e) override;
+    void draw(Canvas& c) override;
+
+   private:
+    enum class Page { Menu, Text, Editor };
+    bool onMenuKey(const KeyEvent& e);
+    void notify(const char* text, NotificationLevel level);
+
+    EventBus& bus_;
+    Page page_ = Page::Menu;
+    ListModel menu_{theme::kContent.h / theme::kLineHeight};
+    int textScroll_ = 0;
+    int textLineCount_ = 0;
+    LineEditor editor_{39};
+    std::unique_ptr<DialogModel> dialog_;
+};
+
+}  // namespace roro