Draft Modpack - Compatibility based instance creation

[Yuca-lx]

Please confirm the following.

• I checked the existing issues for duplicate feature requests
• I have checked that this feature request is not on our roadmap

What parts of Modrinth is your feature request related too?

Modrinth App

Is your suggested feature related to a problem? Please describe.

When creating a new instance in the Modrinth App, you are required to pick a game version and loader before selecting any mods. This is a backwards workflow for users (like me) who already have a specific set of mods in mind. They have no way of knowing upfront which version and loader actually supports all of them.

Additionally, while Collections exist on the Modrinth website, they are currently not usable in the app at all (see #1166). This means users who have a collection of mods on the web have no way to use it in one go when setting up a new instance.

Describe the solution you'd like

A new "Draft Modpack" option in the Create Instance dialog, allowing users to select mods first and automatically find the most compatible game version and loader.

Core features:

1. Compatibility finder: As mods are added to the draft, the app calculates which MC version + loader combinations are supported by all selected mods. If no perfect match exists, it shows partial results (e.g. "19/20 mods supported on Fabric 1.21.1") so the user can make a decision based on what they prefer to have.

2. Pinned mods: Users can mark specific mods as "required", meaning the compatibility search always prioritizes combinations where those mods work, even if it means some others don't.

Optional: Collections as a starting point: Users should be able to start a Draft Modpack from an existing Modrinth Collection, with the app immediately calculating the best version/loader for that collection. This would also close the gap of Collections being unusable in the app.

I made an example of how I thought it could maybe look in the instance creation:

Describe alternatives you've considered

• Manually checking each mod's supported versions before creating an instance
• Using the existing Collections on the website as a reference, but there is no way to interact with them in the app (see Integrate collections on the app! #1166) and know if they all work together
• The "Install modpack" option, but this only works for pre-built modpacks, not for users who want to build their own from scratch or just want to have some mods on their "watchlist". If mods get updated so that in the future they would be compatible it wouldn't work with a static instance

Additional context

Related issues:

• Integrate collections on the app! #1166 — Collections not accessible in the app
• Collections of mods that you can add into a modpack #5786 — Collections of mods that you can add into a modpack