Implement semester-based quarterly tracking system

- Update quarterly confirmation deadlines to semester-based schedule:
  - Q1: March 15 (covers Spring semester Q1+Q2)
  - Q2: June 15 (auto-approved when Q1 approved)
  - Q3: September 15 (covers Fall semester Q3+Q4)
  - Q4: December 15 (auto-approved when Q3 approved)

- Add auto-approval functionality:
  - Q1 approval automatically approves Q2 with same document status
  - Q3 approval automatically approves Q4 with same document status
  - New 'auto_geprueft' status with distinct badge UI

- Maintain quarterly payment cycle while simplifying document submissions
- Remove modal edit functionality, keep full-screen editor only
- Update copilot instructions documentation

Changes align with academic semester system where students submit
documents twice yearly instead of quarterly.

app/stiftung/migrations/0035_update_semester_deadlines.py

@@ -0,0 +1,13 @@
+# Generated by Django 5.0.6 on 2025-09-30 19:29
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0034_add_auto_geprueft_status'),
+    ]
+
+    operations = [
+    ]