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.cpp

@@ -0,0 +1,24 @@
+#include "screen.h"
+
+#include <M5Cardputer.h>
+
+namespace roro {
+
+bool Screen::begin() {
+    canvas_.setColorDepth(16);
+    canvas_.setPsram(false);
+    return canvas_.createSprite(theme::kWidth, theme::kHeight) != nullptr;
+}
+
+void Screen::render(AppManager& apps, const StatusInfo& status, const Toast* toast) {
+    canvas_.fillSprite(theme::kBackground);
+    const auto& area = theme::kContent;
+    canvas_.setClipRect(area.x, area.y, area.w, area.h);
+    apps.foreground().draw(canvas_);
+    canvas_.clearClipRect();
+    widgets::statusBar(canvas_, status);
+    if (toast) widgets::toast(canvas_, *toast);
+    canvas_.pushSprite(&M5Cardputer.Display, 0, 0);
+}
+
+}  // namespace roro