fix: dev server intercepts Vite URLs when basePath is configured

[bartlomieju]

Summary

When base: "/ui/" is set in vite.config.ts, the Fresh dev server middleware intercepted Vite's virtual module URLs (like /ui/@id/fresh:client-entry) instead of passing them to Vite's middleware, causing 404s for client-entry JS and island code.

Root cause

The IGNORE_URLS regex was hardcoded as /^\/(@(vite|fs|id)|\.vite)\//, which only matches URLs starting with /@id/, /@vite/, etc. When Vite's base is set to /ui/, virtual module URLs become /ui/@id/... which didn't match.

Fix

Build the regex dynamically from server.config.base, matching both /@id/... and /ui/@id/...:

const base = server.config.base.replace(/\/$/, "");
const IGNORE_URLS = new RegExp(`^(${base})?/(@(vite|fs|id)|\\.vite)/`);

Test plan

• Verified /ui/@id/fresh:client-entry returns 200 (was 404)
• Verified /ui/@vite/client returns 200 (was 404)
• Verified /@id/... without base still works (backward compatible)

Closes #3666

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