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/list_model.cpp

@@ -0,0 +1,41 @@
+#include "list_model.h"
+
+namespace roro {
+
+void ListModel::setCount(int count) {
+    count_ = count < 0 ? 0 : count;
+    if (count_ == 0) {
+        selected_ = -1;
+        first_ = 0;
+        return;
+    }
+    select(selected_ < 0 ? 0 : selected_);
+}
+
+void ListModel::select(int index) {
+    if (count_ == 0) return;
+    selected_ = index < 0 ? 0 : index >= count_ ? count_ - 1 : index;
+    scrollToSelection();
+}
+
+void ListModel::up() {
+    if (count_ == 0) return;
+    select(selected_ == 0 ? count_ - 1 : selected_ - 1);
+}
+
+void ListModel::down() {
+    if (count_ == 0) return;
+    select(selected_ == count_ - 1 ? 0 : selected_ + 1);
+}
+
+void ListModel::pageUp() { select(selected_ - rows_); }
+void ListModel::pageDown() { select(selected_ + rows_); }
+
+void ListModel::scrollToSelection() {
+    if (selected_ < first_) first_ = selected_;
+    if (selected_ >= first_ + rows_) first_ = selected_ - rows_ + 1;
+    int maxFirst = count_ > rows_ ? count_ - rows_ : 0;
+    if (first_ > maxFirst) first_ = maxFirst;
+}
+
+}  // namespace roro