Allow setting API, proxy, and frontend URLs at build time + support subpaths

[fkrull]

I guess this is sorta two features in one:

• I replaced most occurrences of hardcoded URLs in the code with references to environment variables that can be set at build time.
  • I left out opensearch.xml because I think expanding variables in that file would've required an additional vite plugin and I didn't want to mess with the build to that degree.
• Make it so vite's base setting can be used to run the frontend in a subpath rather than on the toplevel of a domain. This one is a bit of a best effort: I tested it and everything I tried worked, but I'm not sure I fixed every link that might ignore the subpath. But the changes shouldn't (fingers crossed) make any difference for the default "/" base URL.

[sweep-ai[bot]]

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