minor and major reworks: rename swb to SRU, add a test for pdf parsing

major: rework mail to send mail as plaintext instead of html, preventing the bleed-in of html text

src/ui/widgets/graph.py

@@ -8,7 +8,7 @@ from PySide6.QtCharts import QCategoryAxis, QChart, QChartView, QLineSeries, QVa
 from PySide6.QtGui import QColor, QPainter, QPen
 
 from src import LOG_DIR
-from src.backend.semester import Semester
+from src.logic.semester import Semester
 
 log = loguru.logger
 log.remove()