JComic: update to 1.6

[hualiong]

For this source, implementing HttpSource is more convenient than KeiSource.

Checklist:

• Updated versionCode value in build.gradle.kts
• Updated baseVersionCode in build.gradle.kts (if updated multisrc theme code)
• Referenced all related issues in the PR body (e.g. "Closes #xyz")
• Set the contentWarning configuration in build.gradle.kts appropriately
• Have not changed source names
• Have explicitly kept the id if a source's name or language were changed
• Have tested the modifications by compiling and running the extension through Android Studio
• Have removed web_hi_res_512.png when adding a new extension
• This PR is AI-assisted, I have reviewed the changes manually and confirmed they are not slop

[AwkwardPeak7]

same here, please implement KeiSource; also same comment for client.get getting headers and .string helpers instead of .jsonPrimitive.content etc