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

frontend/src/i18n/de.json

@@ -183,6 +183,13 @@
     "notes": "Notizen",
     "medicationImage": "Medikamentenbild",
     "removeImage": "Bild entfernen",
+    "imageUploadErrors": {
+      "tooLarge": "Das Bild ist zu groß. Die maximale Upload-Größe beträgt 10 MB.",
+      "invalidType": "Ungültiger Dateityp. Erlaubte Formate: JPEG, PNG, WebP, GIF.",
+      "invalidImage": "Ungültige oder nicht unterstützte Bilddatei.",
+      "noFile": "Es wurde keine Datei zum Hochladen ausgewählt.",
+      "generic": "Bild-Upload fehlgeschlagen. Bitte versuche es erneut."
+    },
     "placeholders": {
       "commercial": "z.B. Ozempic",
       "generic": "z.B. Semaglutid (optional)",

frontend/src/i18n/en.json

@@ -183,6 +183,13 @@
     "notes": "Notes",
     "medicationImage": "Medication Image",
     "removeImage": "Remove Image",
+    "imageUploadErrors": {
+      "tooLarge": "Image is too large. Maximum upload size is 10 MB.",
+      "invalidType": "Invalid file type. Allowed formats: JPEG, PNG, WebP, GIF.",
+      "invalidImage": "Invalid or unsupported image file.",
+      "noFile": "No file was selected for upload.",
+      "generic": "Image upload failed. Please try again."
+    },
     "placeholders": {
       "commercial": "e.g. Ozempic",
       "generic": "e.g. Semaglutide (optional)",