refactor(engine): move click annotation from sharp to browser Canvas

The vision pipeline used sharp to draw a click marker on the scene image
server-side (engine/src/annotate.ts) and to render the MOCK_IMAGE
placeholder PNG (engine/src/mockImage.ts). Both moved off the runtime:

- annotateClick → apps/web/lib/annotateClient.ts (Canvas 2D in the
  browser; toDataURL → raw PNG base64 forwarded to /api/vision). Saves
  a server-side image re-fetch per click and frees the engine from
  sharp's native binding (which doesn't run on Cloudflare Workers).
- mockImageDataUri → self-describing SVG data URI (no rendering needed).

VisionRequest contract changes: prevImageUrl + click → annotatedImageBase64.
Server forwards the bytes straight to the vision LLM as image_url.

sharp is removed from packages/engine entirely and from next.config.ts's
serverExternalPackages. apps/web/package.json + lockfile cleanup ships
in the follow-up Cloudflare deployment commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/engine/package.json

@@ -15,7 +15,6 @@
     "@infiplot/ai-client": "workspace:*",
     "@infiplot/tts-client": "workspace:*",
     "@infiplot/types": "workspace:*",
-    "jsonrepair": "^3.14.0",
-    "sharp": "^0.33.5"
+    "jsonrepair": "^3.14.0"
   }
 }