feat(groups): add CSV export for grades, attendance, homework and summary

Adds four CSV exports accessible from the group detail screen via a
new download icon in the AppBar:
- Grades matrix (students × sessions with weighted average column)
- Attendance sheet (students × dates with absence summary)
- Homework & material compliance (two-section sheet with rates)
- Full summary (one row per student with all averages and rates)

Files are saved to the app documents directory. On mobile a share
sheet is also shown via share_plus.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mikebarkmin

assets/translations/de.json

@@ -102,6 +102,13 @@
   "done": "Erledigt",
   "edit": "Bearbeiten",
   "edit_avatar": "Avatar bearbeiten",
+  "export": "Exportieren",
+  "export_grades": "Notenmatrix",
+  "export_attendance": "Anwesenheitsliste",
+  "export_homework_material": "Hausaufgaben & Material",
+  "export_summary": "Vollständige Übersicht",
+  "export_success": "Datei gespeichert",
+  "export_failed": "Export fehlgeschlagen",
   "edit_grade": "Note bearbeiten",
   "edit_grade_system": "Notensystem bearbeiten",
   "edit_homework": "Hausaufgaben bearbeiten",

assets/translations/en.json

@@ -102,6 +102,13 @@
   "done": "Done",
   "edit": "Edit",
   "edit_avatar": "Edit avatar",
+  "export": "Export",
+  "export_grades": "Grades matrix",
+  "export_attendance": "Attendance sheet",
+  "export_homework_material": "Homework & material",
+  "export_summary": "Full summary",
+  "export_success": "File saved",
+  "export_failed": "Export failed",
   "edit_grade": "Edit grade",
   "edit_grade_system": "Edit grade system",
   "edit_homework": "Edit homework",