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/test/components/Auth.test.tsx

@@ -896,7 +896,7 @@ describe("AuthProvider methods", () => {
 		});
 
 		const file = new File(["avatar"], "avatar.png", { type: "image/png" });
-		await expect(result.current.uploadAvatar(file)).rejects.toThrow("Upload failed");
+		await expect(result.current.uploadAvatar(file)).rejects.toThrow("UNKNOWN");
 	});
 
 	it("deleteAvatar succeeds and refreshes user", async () => {

frontend/src/test/hooks/useMedications.test.ts

@@ -170,9 +170,11 @@ describe("useMedications", () => {
 		const { result } = renderHook(() => useMedications());
 		const file = new File(["test"], "test.jpg", { type: "image/jpeg" });
 
-		await act(async () => {
-			await result.current.uploadMedImage(1, file);
-		});
+		await expect(
+			act(async () => {
+				await result.current.uploadMedImage(1, file);
+			})
+		).rejects.toThrow("Upload failed");
 
 		expect(result.current.uploadingImage).toBe(false);
 	});