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/core/src/app.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <cstdint>
+
 #include "key_event.h"
 
 namespace roro {
@@ -20,6 +22,9 @@ class App {
         return false;
     }
 
+    // Called every main-loop pass while in the foreground (e.g. to refresh live values).
+    virtual void update(uint32_t nowMs) { (void)nowMs; }
+
     virtual void draw(Canvas& canvas) = 0;
 
     void requestRedraw() { redraw_ = true; }

lib/core/src/app_manager.h

@@ -27,6 +27,7 @@ class AppManager {
     void home();
 
     void handleKey(const KeyEvent& event);
+    void update(uint32_t nowMs) { foreground_->update(nowMs); }
 
     App& foreground() const { return *foreground_; }