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/widgets.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "canvas.h"
+#include "dialog_model.h"
+#include "line_editor.h"
+#include "list_model.h"
+#include "text_wrap.h"
+#include "theme.h"
+#include "toast_queue.h"
+
+namespace roro {
+
+struct StatusInfo {
+    std::string title;
+    int batteryPercent = -1;  // -1: unknown
+    std::string clock = "--:--";
+    bool sdPresent = false;
+    int sdLevel = 0;  // StorageMonitor level
+    char compose = 0;  // pending Compose Key state
+
+    bool operator==(const StatusInfo& o) const {
+        return title == o.title && batteryPercent == o.batteryPercent && clock == o.clock &&
+               sdPresent == o.sdPresent && sdLevel == o.sdLevel && compose == o.compose;
+    }
+    bool operator!=(const StatusInfo& o) const { return !(*this == o); }
+};
+
+namespace widgets {
+
+// Width in pixels of text in the body font.
+MeasureText bodyMeasure(Canvas& canvas);
+
+void statusBar(Canvas& c, const StatusInfo& info);
+
+// A selectable list; `label(i)` gives row i's text.
+void list(Canvas& c, const ListModel& model, theme::Rect area, const std::function<std::string(int)>& label);
+
+// Pre-wrapped lines starting at `firstLine`, with a scroll indicator when they don't all fit.
+void textLines(Canvas& c, const std::vector<std::string>& lines, int firstLine, theme::Rect area);
+
+// One line of editable text with a cursor; scrolls horizontally to keep the cursor visible.
+void lineEditor(Canvas& c, const LineEditor& editor, theme::Rect area);
+
+void dialog(Canvas& c, const std::string& title, const std::string& message, const DialogModel& model);
+
+void toast(Canvas& c, const Toast& t);
+
+}  // namespace widgets
+}  // namespace roro