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.cpp

@@ -0,0 +1,38 @@
+#include "notifier.h"
+
+#include <M5Cardputer.h>
+
+namespace roro {
+
+namespace {
+constexpr int kLedPin = 21;  // WS2812, powered with the backlight rail (G38)
+}
+
+Notifier::Notifier(EventBus& bus, const Settings& settings) : settings_(settings) {
+    bus.subscribe(EventType::Notification, [this](const Event& e) { present(e); });
+}
+
+void Notifier::present(const Event& e) {
+    auto level = static_cast<NotificationLevel>(e.a);
+    uint32_t now = millis();
+    toasts_.push(e.text, level, now);
+
+    if (!settings_.getBool(Setting::Sound)) return;
+    M5Cardputer.Speaker.tone(level == NotificationLevel::Warning ? 880 : 1760, 60);
+    switch (level) {
+        case NotificationLevel::Warning: rgbLedWrite(kLedPin, 40, 16, 0); break;
+        case NotificationLevel::Message: rgbLedWrite(kLedPin, 0, 40, 0); break;
+        default: rgbLedWrite(kLedPin, 0, 16, 40); break;
+    }
+    ledOn_ = true;
+    ledOffAtMs_ = now + 150;
+}
+
+void Notifier::update(uint32_t nowMs) {
+    if (ledOn_ && static_cast<int32_t>(nowMs - ledOffAtMs_) >= 0) {
+        rgbLedWrite(kLedPin, 0, 0, 0);
+        ledOn_ = false;
+    }
+}
+
+}  // namespace roro