Change prefix to "mokuro_*"

This was an alternative I was thinking of initially since it better namespaces the files, but wasn't sure about.

I saw this commit ended up going with this approach, so I'm aligning the PR to it in the end:

6d550cbe8a

src/lib/anki-connect/index.ts

@@ -102,7 +102,7 @@ export async function updateLastCard(imageData: string | null | undefined, sente
         id,
         fields,
         picture: {
-          filename: `${id}.webp`,
+          filename: `mokuro_${id}.webp`,
           data: imageData.split(';base64,')[1],
           fields: [pictureField],
         },