feat: improve connect screen with interactive setup wizard

[liuxiaopai-ai]

Summary

Closes #6

The current connect screen only shows static instructions telling users to manually create .env.local files and restart the dev server. This is confusing — users don't understand what values to use or that they need to restart.

Changes

New: Interactive connection form

• Clean form with fields for Gateway URL (default: ws://127.0.0.1:18789), Token (recommended), and Password (fallback)
• "Test Connection" button that validates credentials server-side before the user saves anything
• Clear success/error states with actionable feedback
• Auto-generated copy-paste .env.local snippet pre-filled with the user's values
• Explicit restart instructions shown after successful test

New: /api/connect-test endpoint

• POST endpoint that accepts { url, token, password } and tests the gateway connection with those credentials
• Uses a new gatewayConnectTest() helper in gateway.ts — same WebSocket handshake logic but with caller-provided credentials instead of env vars

Improved: Manual setup as collapsible

• Existing manual .env.local instructions moved into a collapsible "Manual setup instructions" section for advanced users
• "Where to find these values" reference card kept at the bottom

Design decisions

• No server architecture changes — the form tests credentials then guides users to save .env.local and restart. Clean and simple.
• Uses existing UI components (Button, Collapsible, CodeBlock) and Tailwind patterns
• Uses @hugeicons/react icons consistent with the rest of the app
• Minimal footprint: 3 files changed, ~350 lines added