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>

packages/types/src/index.ts

@@ -60,12 +60,22 @@ export type StartResponse = {
   imageBase64: string;
 };
 
-export type InteractRequest = {
+export type VisionRequest = {
   session: Session;
   prevImageBase64: string;
   click: { x: number; y: number };
 };
 
+export type VisionResponse = {
+  intent: ClickIntent;
+};
+
+export type InteractRequest = {
+  session: Session;
+  intent: ClickIntent;
+  click?: { x: number; y: number };
+};
+
 export type InteractResponse = {
   session: Session;
   frame: StoryFrame;