feat: App - Support setting base URL during build

[Ark-kun]

This is needed to host OpenCode app under under some URL prefix. Usage: Set VITE_BASE_URL env var during build. This automatically sets the base in Vite config. It's also used in routers.
See https://vite.dev/config/shared-options#base

Issue for this PR

Issue: #7624

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This change is needed to host OpenCode app under under some URL prefix.
This tiny PR makes it possible to build OpenCode Web App with VITE_BASE_URL="/subdir/" and have it run under http://<host>:<port>/subdir/ instead of root (http://<host>:<port>/).

Set VITE_BASE_URL env var during build. This automatically sets the base in Vite config. Vite generates correct URLs in index.html.
import.meta.env.BASE_URL is also used in routers. See https://vite.dev/config/shared-options#base
You can also set VITE_OPENCODE_SERVER_BASE_URL to specify the base URL prefix for the server.

How did you verify your code works?

% VITE_BASE_URL="/XXX" VITE_OPENCODE_SERVER_BASE_URL="/" bun dev
$ vite
(!) "base" option should start with a slash.

  VITE v7.1.4  ready in 320 ms

  ➜  Local:   http://localhost:3000/XXX

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[einarpersson]

If this works, I think this approach seems good as it has very few changes to the codebase and thus feels less brittle than patching everywhere.

I want to host opencode behind Caddy and tailscale (the latter not allowing for subdomains atm).

[solarkraft]

I have tested this and it works great! Same use case, comfortably statically hosting with Caddy now!

[Ark-kun]

Hello, @Brendonovich. and @Hona . Would you please take a look at this PR? It's just couple simple lines of code, but if you have any concerns, our company can set up a video meeting so that we can explain each line.

[Ark-kun]

@thdxr @Brendonovich @Hona @rekram1-node @adamdotdevin Can you please take a look? It's 6 lines of code...

[Ark-kun]

@Brendonovich Thank you for approving my PR.

Thanks for improving the code. Your 1c15cef commit made the code more elegant. However, it changed the behavior slightly (URL.href adds trailing slash:new URL("http://localhost:4096").href == "http://localhost:4096/"). Such trailing slash triggers navigation bugs in consuming code. See the failing tests: https://github.com/anomalyco/opencode/actions/runs/31237532949/job/93052787926?pr=18209
I've fixed this new issue: 755fa7c