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/launcher_app.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "app.h"
+#include "app_manager.h"
+#include "list_model.h"
+#include "ui/theme.h"
+
+namespace roro {
+
+// Home screen: lists the visible Apps; Select opens one.
+class LauncherApp : public App {
+   public:
+    void setManager(AppManager& manager) { manager_ = &manager; }
+    void onEnter() override;
+    bool onKey(const KeyEvent& e) override;
+    void draw(Canvas& c) override;
+
+   private:
+    AppManager* manager_ = nullptr;
+    ListModel list_{theme::kContent.h / theme::kLineHeight};
+};
+
+}  // namespace roro