feat: add account deletion feature (#85)

* feat: add account deletion feature

- Add DELETE /auth/me endpoint to delete user account and all data
- Add deleteAccount() method to AuthContext
- Add Delete Account button with confirmation modal in UserProfile
- Add danger zone styling (.btn-danger, .profile-danger-zone)
- Add i18n translations for EN and DE
- Add backend tests for account deletion endpoint
- Add timeout settings to frontend vitest.config.ts
- Reduce CI timeout for frontend tests (10min -> 5min)

* fix: improve delete account section layout

- Make profile modal scrollable with max-height
- Add proper horizontal margin to danger zone
- Align delete section with form content

* fix: use ConfirmModal component for delete account dialog

- Replace inline modal with existing ConfirmModal component
- Ensures consistent button styling across all modals
- Add UI consistency rule to AGENTS.md and copilot-instructions.md

* fix: consistent styling for delete account section

- Remove warning text (users know what delete means)
- Remove border-bottom from danger zone title (section has border-top)
- Update copilot-instructions and AGENTS.md with stricter UI consistency rules
- Remove unused deleteAccountHint i18n keys

* chore: remove pre-push test hook (CI handles tests)

Tests were running twice - in pre-push hook and GitHub CI.
Removing local pre-push tests since CI provides authoritative test results.
Use 'npm test' manually before pushing if you want local feedback.

frontend/src/i18n/de.json

@@ -309,7 +309,11 @@
     "avatarUpdated": "Avatar aktualisiert",
     "avatarRemoved": "Avatar entfernt",
     "loginWithSSO": "Mit {{provider}} anmelden",
-    "or": "oder"
+    "or": "oder",
+    "deleteAccount": "Konto löschen",
+    "deleteAccountConfirmTitle": "Konto löschen?",
+    "deleteAccountConfirmText": "Dadurch werden dein Konto und alle deine Daten (Medikamente, Einstellungen, Verlauf) dauerhaft gelöscht. Diese Aktion kann nicht rückgängig gemacht werden.",
+    "deleteAccountButton": "Ja, mein Konto löschen"
   },
   "common": {
     "loading": "Wird geladen...",