fix(desktop): make dev server port configuration more flexible

[aspiers]

Without this patch, the desktop application requires a strict port configuration (port 1420) and hardcodes the devUrl in tauri.conf.json. This causes issues when the port is already in use during development.

This is a problem because developers cannot easily run multiple instances or work around port conflicts without manual configuration changes.

This patch solves the problem by removing the hardcoded devUrl from tauri.conf.json, setting strictPort to false in vite.config.ts to allow Vite to find an available port, and updating the Rust code to read the dev URL from the Tauri config or fall back to the default localhost:1420.

Closes #6943.