feat: prefetch, vision split, provider adapter, UI polish

Engine
- Split /api/vision out from /api/interact so client can drive
  prefetch + cache lookup independently of click interpretation
- Image client switched to chat-completions+modalities API (OpenRouter/
  provider style), supporting markdown image URL responses
- annotateClick now resizes to 768w before composite to keep vision
  payloads small and avoid CDN timeouts
- Prompts updated to mention "JSON" in user messages (required by
  Gemini's strict JSON mode)
- Shared fetchWithRetry helper: 2 retries for chat/image, 0 for vision
  (with 60s hard timeout)

Client
- Parallel prefetch of all three choice branches on each new frame
- Effect deliberately excludes phase from deps so user-click doesn't
  abort in-flight prefetches
- Cache hit/miss/free-form fallback handled in handleClick
- PlayCanvas reads img naturalWidth/Height and adapts container to
  whatever aspect AI returns (no more cropped third choice)
- max-width raised to 560px, max-height calc(100dvh - 200px)

Misc
- README env-path corrected to apps/web/.env.local
- users.md: BGM/TTS idea note
- .env.example moved into apps/web alongside next config

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -49,7 +49,7 @@ Three providers, all independently configurable. Any OpenAI-compatible chat / im
 | Image · UI renderer   | `IMAGE_BASE_URL` `IMAGE_API_KEY` `IMAGE_MODEL` | `gpt-image-2` via OpenAI |
 | Vision · click reader | `VISION_BASE_URL` `VISION_API_KEY` `VISION_MODEL` | `gemini-3-flash` via Google |
 
-See `.env.example` for the exact shape.
+See `apps/web/.env.example` for the exact shape.
 
 ---
 
@@ -59,7 +59,7 @@ Requires Node 20+ and pnpm 9+.
 
 ```bash
 pnpm install
-cp .env.example .env.local
+cp apps/web/.env.example apps/web/.env.local
 # fill in the nine env vars
 pnpm dev
 # open http://localhost:3000