[Bug] ApiUtils.md5hash discards leading 0s on hash (AKA: login failures via Fever integration)

[campbellr]

Describe the bug

When a user happens to be unlucky enough to have Fever credentials where the username + ":" + password happen to result in an MD5 hash with leading zeros, they are unable to log in via the Readrops app.

After a bit of digging, this seems to be because of an incorrect hash implementation in ApiUtils.md5hash:

https://github.com/readrops/Readrops/blob/9ebbe038d0b1357be1a3713dd3222f51820c7f45/api/src/main/java/com/readrops/api/utils/ApiUtils.kt#L47-L51)

It turns out that BigInteger.toString(16) doesn't preserve leading zeros, so an invalid API key is generated, and the user cannot login.

I believe the following diff would fix it, but I'm not familiar with Kotlin, but it works in this kotlin playground example:

diff --git a/api/src/main/java/com/readrops/api/utils/ApiUtils.kt b/api/src/main/java/com/readrops/api/utils/ApiUtils.kt
index c879c56a..52d0b931 100644
--- a/api/src/main/java/com/readrops/api/utils/ApiUtils.kt
+++ b/api/src/main/java/com/readrops/api/utils/ApiUtils.kt
@@ -48,7 +48,7 @@ object ApiUtils {
         val bytes = MessageDigest.getInstance("MD5")
                 .digest(value.toByteArray())
 
-        return BigInteger(1, bytes).toString(16)
+        return BigInteger(1, bytes).toString(16).padStart(32, '0')
     }
 
     fun handleRssSpecialCases(url: String): String {

To Reproduce Steps to reproduce the behavior:

1. Create a set of Fever credentials in Miniflux that result in a hash with leading zeros (eg: user4013:pass4013 which hashes to 0003296c0fa9a2bad56701b3fff82f21
2. Attempt to set up a new Fever account in Readrops with the same credentials
3. Clicking validate will fail
4. See error

Expected behavior

I expect to be able to log in

Environment information (please complete the following information):

• Account type: Fever (via Miniflux, in my case)
• App version: 2.1.1
• Android version: Android 16
• Device type: Pixel 8a
• Store: F-Droid

Additional context Add any other context about the problem here.