fix(sync): clarify WebDAV sync folder scope

[codedogQBY]

Analysis

Issue #355 is caused by a mismatch between user expectations and the intended WebDAV sync model. The sync remote folder is a ReadAny-owned bidirectional sync directory and is not a library import path. Users who already have books in WebDAV should use the WebDAV import flow instead of pointing sync at the WebDAV root.

Changes

• Clarify syncRemoteRootDesc in all settings locales.
• Explain that the folder is reserved for ReadAny sync and does not scan existing remote books.
• Point users to "Connect another WebDAV" for importing an existing WebDAV library.

Verification

• node -e "for (const f of ['en','zh','zh-TW','ja','ko','fr','es']) JSON.parse(require('fs').readFileSync('packages/core/src/i18n/locales/'+f+'/settings.json','utf8')); console.log('settings locale JSON ok')"
• git diff --check
• pnpm --filter @readany/core exec tsc --noEmit
• pnpm --filter app exec tsc --noEmit
• pnpm --filter @readany/app-expo exec tsc --noEmit

Fixes #355