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

lib/ui/src/toast_queue.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <cstdint>
+#include <deque>
+#include <string>
+
+#include "event.h"
+
+namespace roro {
+
+struct Toast {
+    std::string text;
+    NotificationLevel level;
+};
+
+// Notifications waiting to be shown as Toasts, one at a time, each for the same duration.
+class ToastQueue {
+   public:
+    explicit ToastQueue(uint32_t durationMs, size_t capacity = 4) : durationMs_(durationMs), capacity_(capacity) {}
+
+    void push(const std::string& text, NotificationLevel level, uint32_t nowMs);
+
+    // The Toast to show now, or nullptr. Advances the queue as Toasts expire.
+    const Toast* current(uint32_t nowMs);
+    void dismiss(uint32_t nowMs);
+
+    // True once after the visible Toast changed (appeared, switched or disappeared).
+    bool takeChanged();
+
+   private:
+    uint32_t durationMs_;
+    size_t capacity_;
+    std::deque<Toast> queue_;
+    bool showing_ = false;
+    uint32_t shownAtMs_ = 0;
+    bool changed_ = false;
+};
+
+}  // namespace roro