Add custom URL support with mirror selection

[dejavui]

Introduce CustomAndMirrorPreferences, allowing extensions to combine predefined mirrors with a custom base URL option.

This generalizes the existing MoeTruyen implementation into a reusable framework component, so extensions no longer need to implement their own mirror/custom URL preference logic.

Changes

• Added CustomAndMirrorPreferences to handle mirror selection and custom base URLs in a single preference component.
• Extended BaseUrlSpec.Mirrors with optional entries, values, and withCustom support for declarative mirror configuration.
• Updated SourceProcessor to generate CustomAndMirrorPreferences when custom URLs are enabled, while preserving the existing MirrorPreferences path for regular mirror-only sources.
• Migrated the MoeTruyen extension to the new declarative @Source configuration.
• Removed the manual mirror/custom URL preference implementation from MoeTruyen, as it is now generated by the framework.

Review needed 👉ﾟヮﾟ👉 from @AwkwardPeak7, @vetleledaal, and @FourTOne5. Thanks!

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]

I feel like mirrors and custom urls should remain mutually exclusive.

[dejavui]

I feel like mirrors and custom urls should remain mutually exclusive.

I understand the concern. My intention isn't to merge two unrelated features, but to model different ways of resolving the effective base URL ＼（〇_ｏ）／

MoeTruyen has been using this workflow for a time: users choose either the default domain, one of the predefined mirrors, or a custom URL. Only one option is active at a time, so they're still mutually exclusive from the user's perspective.

[FourTOne5]

if this were to be considered i would expect the custom and mirrors option to be merged