fix(canvas): escape data.json before inlining it into the exported <script>

[245678000000]

Summary

buildSingleFileHtml in Cursor++/src/server/handlers/canvas/canvasStore.ts interpolates the canvas data.json straight into an inline <script> block:

data: new Map(Object.entries(${dataJson || '{}'})),

An HTML parser ends a script block at the first </script, regardless of whether it sits inside a JS string literal. data.json is user/model content — it arrives as DashboardService.ShareCanvas's data_json (services/account/DashboardService.ts:115) — so any string value containing </script> truncates the exported file and everything after it is parsed as DOM.

The sibling title in the same template already goes through escapeHtml; this path was missed.

Repro

const s = '{"a":"</script><h1>pwn</h1>"}';
'<script>window.x = new Map(Object.entries(' + s + '));</script>'
// -> <script>window.x = new Map(Object.entries({"a":"</script><h1>pwn</h1>"}));</script>
//                                                     ^ script block ends here

Fix

Escape < as the JSON escape \u003c before interpolating, via a small helper next to escapeHtml. In JSON < can only occur inside a string literal, so the escape is semantically identical — the data still parses back to exactly the same value. bundleBase64 needs no change (base64 has no <), and buildIndexHtml doesn't interpolate data at all.

Test plan

New Cursor++/src/server/tests/canvasExport.test.ts — round-trip of ordinary data, no new </script from a hostile payload, and value preservation after escaping. Two of the three fail without the source change (verified by stashing the fix).

• pnpm --dir "Cursor++" run check-types
• pnpm --dir "Cursor++" run lint
• pnpm --dir "Cursor++" run test:server — 39 files / 370 tests passed (baseline on main is 38 / 367)

Those counts are from a machine with Cursor installed and a populated ~/.ccursor. On a clean checkout 5 of the 38 files fail for host-dependency reasons that have nothing to do with this change — 4 need @vscode/sqlite3 out of an installed Cursor, and protocol.test.ts:232 reads the real ~/.ccursor/knowledge-base.json. Detail in #20.