feat: image upload optimization with sharp, thumbnails, and structured error codes (#304)

- Add sharp for server-side image processing (WebP conversion + thumbnails)
- New shared backend utility for image upload, optimization, and cleanup
- Return structured error codes from upload endpoints (IMAGE_TOO_LARGE, INVALID_TYPE, etc.)
- Frontend error code mapping with i18n support (EN + DE)
- MedicationAvatar tries thumbnail first, falls back to full image
- Error display in MedicationsPage, MobileEditModal, and Auth avatar upload

Closes #302

backend/package.json

@@ -32,6 +32,7 @@
     "fastify": "^5.7.4",
     "nodemailer": "^8.0.1",
     "openid-client": "^6.8.2",
+    "sharp": "^0.34.5",
     "zod": "^3.23.8"
   },
   "devDependencies": {