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/hooks/useMedications.ts

@@ -50,13 +50,33 @@ export function useMedications(): UseMedicationsReturn {
 					body: formData,
 					credentials: "include",
 				});
-				if (res.ok) {
-					loadMeds();
+				if (!res.ok) {
+					let code = "UNKNOWN";
+					try {
+						const errorBody = (await res.json()) as { code?: string };
+						if (typeof errorBody?.code === "string" && errorBody.code.trim().length > 0) {
+							code = errorBody.code;
+						}
+					} catch {
+						// Keep fallback code when backend response has no JSON body.
+					}
+					throw new Error(code);
 				}
-			} catch {
-				// ignore
+
+				loadMeds();
+			} catch (error) {
+				if (error instanceof Error) {
+					// Network failures (fetch itself throws) produce browser-specific messages.
+					// Normalise to NETWORK_ERROR code so the UI can map to a translated string.
+					if (error.message === "Failed to fetch" || error.message.startsWith("NetworkError")) {
+						throw new Error("NETWORK_ERROR");
+					}
+					throw error;
+				}
+				throw new Error("UNKNOWN");
+			} finally {
+				setUploadingImage(false);
 			}
-			setUploadingImage(false);
 		},
 		[loadMeds]
 	);