C3: Replace /onboard template with preview design

[nextlevelshit]

Parent

Epic #1624
Depends on: #1627 (C1)

Scope

Replace the current /onboard template with the preview mockup design from templates/preview/onboard.html. Preserve all SSE wiring and prompt rendering — only the visual shell changes.

Current state

• templates/onboard/index.html — uses layout.html sidebar, SSE-based chat with prompt forms
• _message.html + _prompt.html partials for chat messages
• SSE stream at /onboard/{sessionID}/stream — events: message, prompt, status, done, error
• Prompt form POSTs to /onboard/{sessionID}/answer

Target state

• New templates/onboard/index.html — standalone page using preview design
  • Top nav from C1 (Onboarding active)
  • Page header: "Setting up Wave for this project" + Restart/Abort buttons
  • Stepper: detect → inspect → propose → scaffold → smoke-test → commit
  • Chat area with message types: system, agent, prompt, user
  • Prompt rendering: choice grid (radio buttons) and text input (existing JS logic)
  • Agent message stream output (file write log)
• Keep ALL existing JS/SSE logic — wrap in preview visual shell
• No handler changes

Critical: preserve SSE behavior

The existing JS must continue working:

• EventSource('/onboard/{sessionID}/stream')
• Events: message, prompt, status, done, error
• Prompt form POST with CSRF token
• Auto-scroll conversation
• Reconnect on error

Acceptance criteria

• /onboard/{sessionID} renders with preview design (nav, stepper, chat)
• SSE streaming works — messages appear in chat bubbles
• Prompt rendering works — choice grids and text inputs
• Form submission works — answer POSTs correctly
• Reload survives via Last-Event-ID
• Visual smoke in chromium: chat flow renders correctly

Files touched

• internal/webui/templates/onboard/index.html (rewrite)
• internal/webui/templates/onboard/_message.html (may adapt or inline)
• internal/webui/templates/onboard/_prompt.html (may adapt or inline)

NOT touched

• internal/webui/handlers_onboard.go (no changes)
• SSE endpoints (unchanged)

[nextlevelshit]

Blocked by #1627

[nextlevelshit]

Merged in PR #1633. Onboard + proposals + work pages migrated to unified design system. All standalone pages now use w-* CSS classes from style.css.