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/notifier.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "event_bus.h"
+#include "settings.h"
+#include "toast_queue.h"
+
+namespace roro {
+
+// Presents Notifications: a Toast, plus a short beep and LED flash (when Sound is on).
+class Notifier {
+   public:
+    Notifier(EventBus& bus, const Settings& settings);
+    void update(uint32_t nowMs);  // turns the LED off after the flash
+    ToastQueue& toasts() { return toasts_; }
+
+   private:
+    void present(const Event& e);
+
+    const Settings& settings_;
+    ToastQueue toasts_{3000};
+    uint32_t ledOffAtMs_ = 0;
+    bool ledOn_ = false;
+};
+
+}  // namespace roro