Importing a skill from the cloud market failed with
'SyntaxError: No number after minus sign in JSON at position 1 (line 1 column 2)'
because the WebPlatformAdapter hard-coded res.json() for every response, while
the same IPC handler on Electron already routes by Content-Type / .json suffix
(see apps/desktop/main/ipc/window.ts: '支持 JSON 和纯文本/Markdown').
The Electron path handled skill bodies (.md) correctly; the Web path only
worked for the catalog (skill.json). Aligning the Web adapter with the same
Content-Type / .json-suffix rule fixes both fetchCloudCatalog and
importFromCloud at once, with no new abstraction.
