fix: skip electron plugin in CI to prevent X11 display error

The vite-plugin-electron was trying to spawn Electron during the Vite
dev server startup, which fails in CI because there's no X11 display.

- Use Vite's function config to check command type (serve vs build)
- Only skip electron plugin during dev server (command=serve) in CI
- Always include electron plugin during build for dist-electron/main.js
- Add VITE_SKIP_ELECTRON env var support for explicit control
- Update playwright.config.ts to pass VITE_SKIP_ELECTRON in CI

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/playwright.config.ts

@@ -52,6 +52,8 @@ export default defineConfig({
             env: {
               ...process.env,
               VITE_SKIP_SETUP: "true",
+              // Skip electron plugin in CI - no display available for Electron
+              VITE_SKIP_ELECTRON: process.env.CI === "true" ? "true" : undefined,
             },
           },
         ],