i18n(ja): fix the "thread"/"yarn" mistranslation

[yahonda]

What is changed, added or deleted? (Required)

The old machine-translation pipeline rendered English "thread" as 糸 (the literal Japanese word for thread/yarn), which is wrong in these technical contexts:

• system-variables.md (16 places): Unit: Threads was translated as 単位：糸 → corrected to 単位：スレッド. Affects concurrency variables such as tidb_distsql_scan_concurrency, tidb_build_stats_concurrency, etc.
• tidb-cloud/data-service-oas-with-nextjs.md (1 place): the link text for the yarn package manager was translated to [糸] → restored to [yarn].

No English/source content changed; only the Japanese rendering.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)

What is the related PR or file link(s)?

N/A — cleanup of legacy MT artifacts on the ja translation branch.

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