fix(monaco): whitelist the monaco-write path the dev server actually resolves

[NgoQuocViet2001]

Problem

resolveSnippetImport returns two different things:

const src = slash(
  filepath.startsWith('@/')
    ? path.resolve(userRoot, filepath.slice(2))
    : path.resolve(dir, filepath),      // dir = dirname(slide.source.filepath)
)
...
return { content, filepath, lang, meta, src }

filepath is the specifier exactly as typed in the Markdown; src is where it actually lives. Reading uses src. The {monaco-write} branch used filepath:

monacoWriterWhitelist.add(filepath)
...
const safeFilepath = JSON.stringify(filepath).slice(1, -1)
token.content = `<Monaco writable="${safeFilepath}" ... />`

and createMonacoWriterPlugin resolves whatever it receives against userRoot:

const filepath = path.resolve(userRoot, file)

So the snippet is read through one resolution scheme and written back through another.

snippet	read from	written to
<<< @/snippets/x.ts {monaco-write}	<userRoot>/snippets/x.ts	<userRoot>/@/snippets/x.ts
<<< ./x.ts {monaco-write} in pages/deck.md	<userRoot>/pages/x.ts	<userRoot>/x.ts

The @/ form — the one the docs recommend — writes into a directory literally named @, which does not exist, so fs.writeFile rejects with ENOENT inside the websocket handler and the save is lost with no feedback in the editor. The relative form silently writes to a different file than the one on screen.

Fix

Hand the writer the userRoot-relative form of the resolved path, which is the scheme it resolves in:

const writePath = slash(path.relative(userRoot, src))
monacoWriterWhitelist.add(writePath)

monacoWrite.ts and Monaco.vue are untouched. The plugin's rel.startsWith('..') guard keeps working and becomes meaningful — it now compares a path that really was resolved against userRoot.

Test plan

• Added a case to the existing packages/slidev/node/syntax/snippet.test.ts, reusing its test/fixtures/ root and snippets/snippet.ts fixture. It asserts the whitelisted value, the writable attribute, and that path.resolve(userRoot, whitelisted) lands back on the file that was read — the writer's own expression.
• Ran: vitest run packages/slidev/node/syntax/snippet.test.ts → 6 passed.
• Checked: reverting only snippet.ts fails the new test with [ '@/snippets/snippet.ts' ] instead of [ 'snippets/snippet.ts' ].
• Ran: eslint on both touched files → clean.

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commit: 32c009a