Add site-agnostic pob://code/ scheme for inline build imports

### Check for duplicates - [x] I've checked for duplicate open **and closed** issues by using the search function of the [issue tracker](https://github.com/PathOfBuildingCommunity/PathOfBuilding/issues?q=is%3Aissue) ### What platform are you running Path of Building on? Linux - Rusty PoB ### Is your feature request related to a problem? Every pob:// route today requires the build to live on a third-party paste site (pobb.in, maxroll, etc). Tools and scripts that already hold a build code locally have to upload it before they can offer a one-click "Open in PoB" link, which adds an external dependency, an HTTP roundtrip, and it seems unfair to me in general. ### Describe the solution you'd like Add a site-agnostic `pob://code/<base64url-code>` route. The path segment IS the build code — PoB skips the HTTP fetch and decodes it directly via the existing Inflate/base64 path in Main.lua. It looks like this on the target website: <img width="848" height="363" alt="Image" src="https://github.com/user-attachments/assets/465e4cba-9900-4590-8be8-857bcd4fed76" /> ### Describe alternatives you've considered _No response_ ### Additional context I have a patch + tests ready in the PR #9815