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/line_editor.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace roro {
+
+// Single-line UTF-8 text editing. The cursor is a byte index always on a character boundary,
+// and the text never exceeds maxBytes (protocol fields are limited in bytes, not characters).
+class LineEditor {
+   public:
+    explicit LineEditor(size_t maxBytes) : max_(maxBytes) {}
+
+    void setText(const std::string& text);  // truncates, cursor to end
+    const std::string& text() const { return text_; }
+    size_t cursor() const { return cursor_; }
+    size_t maxBytes() const { return max_; }
+
+    bool insert(uint32_t codePoint);  // false if it would not fit
+    void backspace();
+    void left();
+    void right();
+    void home() { cursor_ = 0; }
+    void end() { cursor_ = text_.size(); }
+
+   private:
+    size_t max_;
+    std::string text_;
+    size_t cursor_ = 0;
+};
+
+}  // namespace roro