Fix PR #53: cppcheck CI failure, markup injection, code quality issues

[Copilot]

Review of PR #53 (GTK4 MVC architecture pattern demo) surfaced a CI-blocking cppcheck error and several code quality issues.

CI fix

• Initializer list order in DisplayWidget.cpp and EditorWidget.cpp — members were initialized out of declaration order, triggering cppcheck [initializerList] with --error-exitcode=1

Security

• Pango markup injection — user input was concatenated directly into markup strings. Now escaped via Glib::Markup::escape_text():

auto escaped = Glib::Markup::escape_text(text);
std::string markup = "<span foreground='" + color_ +
                     "' size='x-large' weight='bold'>" + escaped + "</span>";

Code quality

• Add missing #include <string> in IObserver.h (was relying on transitive includes)
• Add missing #pragma once in DisplayWidget.h
• Remove unused #include "../controller/Controller.h" from DisplayWidget.h
• Remove debug std::cout statements and redundant controller_ = c re-assignment in EditorWidget.cpp
• Add missing newline at end of all new files

Documentation

• Fix swapped pros/cons labels and typos ("Cos"→"Pros", "Pos"→"Cons") in src/ap/README.md

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