Refactor: Consolidate sidebar navigation into 5 top-level sections

Per TODO.md navigation consolidation requirements:
- Home: Dashboard, Notifications
- Teach: Classes, Progress, Assessments, Grading, Reports, Attendance, Homework, Lesson Plans, Resources, Notebooks, Drawing, Portfolio, Rubrics, Comments, Peer Assessment, Tablet Grading, Seating, Subjects, Competitions
- School Life: Calendar, Communication, Announcements, Events, Newsletter, Library, Transport, Counseling, Wellness, Career, Discipline, Behavior, Illness, Substitutes, Timetable, Exam Calendar, Parents
- Insights: Competence Flower, Mastery Matrix, Analytics, Coverage, Grade Analytics, Reports, AI Tests, Data Import/Export
- Administration: Competencies, Districts, Settings

Added i18n keys for new section labels (DE + EN).

src/lib/i18n.ts

@@ -813,6 +813,11 @@ const de: TranslationDict = {
   'polish.search_placeholder': 'Schüler, Klassen oder Kompetenzen suchen…',
   'polish.no_results': 'Keine Ergebnisse',
   'polish.help': 'Hilfe',
+  'polish.nav_home': 'Start',
+  'polish.nav_teach': 'Unterricht',
+  'polish.nav_school_life': 'Schulleben',
+  'polish.nav_insights': 'Erkenntnisse',
+  'polish.nav_administration': 'Verwaltung',
   'polish.nav_setup': 'Einrichtung',
   'polish.nav_teaching': 'Unterricht',
   'polish.nav_analysis': 'Analyse',
@@ -7462,6 +7467,11 @@ const en: TranslationDict = {
   'polish.search_placeholder': 'Search students, classes or competencies…',
   'polish.no_results': 'No results',
   'polish.help': 'Help',
+  'polish.nav_home': 'Home',
+  'polish.nav_teach': 'Teach',
+  'polish.nav_school_life': 'School Life',
+  'polish.nav_insights': 'Insights',
+  'polish.nav_administration': 'Administration',
   'polish.nav_setup': 'Setup',
   'polish.nav_teaching': 'Teaching',
   'polish.nav_analysis': 'Analysis',