desktop: normalize local backend URL and add /api dev proxy

[roerohan]

Fixes #5928

Summary

I've fixed the issue where the desktop app couldn't properly connect to the API server when running with a custom port. Here's what I changed:

Changes Made:

1. packages/desktop/src/app.tsx - Fixed the URL detection logic to use 127.0.0.1:${port} for localhost instead of trying to use the Vite dev server port
2. packages/desktop/vite.config.ts - Added optional proxy configuration for better development experience

How I tested:

Default Port (4096):

• Terminal 1

cd packages/opencode && bun src/index.ts

• Terminal 2

cd packages/desktop && bun run dev

• Open http://localhost:3000

Custom Port (e.g., 8080):

• Terminal 1

cd packages/opencode && bun src/index.ts --port=8080

• Terminal 2

cd packages/desktop && VITE_OPENCODE_SERVER_PORT=8080 bun run dev

• Open http://localhost:3000

The desktop app will now correctly connect to the API server on whatever port you specify via the VITE_OPENCODE_SERVER_PORT environment variable, which defaults to 4096.

[Brendonovich]

#5928 is fixed now