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.h

@@ -0,0 +1,32 @@
+#pragma once
+
+namespace roro {
+
+// Selection and scrolling for a vertical list showing `visibleRows` rows at a time.
+// Up/Down wrap around; page moves don't.
+class ListModel {
+   public:
+    explicit ListModel(int visibleRows) : rows_(visibleRows) {}
+
+    void setCount(int count);
+    int count() const { return count_; }
+    int selected() const { return selected_; }  // -1 when empty
+    int firstVisible() const { return first_; }
+    int visibleRows() const { return rows_; }
+
+    void select(int index);
+    void up();
+    void down();
+    void pageUp();
+    void pageDown();
+
+   private:
+    void scrollToSelection();
+
+    int rows_;
+    int count_ = 0;
+    int selected_ = -1;
+    int first_ = 0;
+};
+
+}  // namespace roro