Add BoundingBoxDialog and map canvas

[jremillard]

Summary

• implement MapCanvas widget for tile display and box selection
• add BoundingBoxDialog to wrap MapCanvas
• expose coordinate conversion helpers in maputils
• update example usage in README
• extend unit tests for new utilities

Testing

• cmake --build bin/release -j$(nproc)
• cmake --build bin/debug -j$(nproc)
• cmake --build bin/coverage -j$(nproc)
• cmake --build bin/valgrind -j$(nproc)
• ctest --test-dir bin/release
• ctest --test-dir bin/valgrind
• ctest --output-on-failure --test-dir bin/coverage
• valgrind --tool=memcheck bin/valgrind/tests/hello_test
• valgrind --tool=helgrind bin/valgrind/tests/hello_test
• lcov --capture --directory bin/coverage --output-file bin/coverage/coverage.info
• lcov --remove bin/coverage/coverage.info '/usr/*' '*/tests/*' --output-file bin/coverage/coverage.info
• lcov --list bin/coverage/coverage.info | sort -k2 > tests/coverage_report.txt

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https://chatgpt.com/codex/tasks/task_e_686a068a30bc833093ab1a066a7c7f80