i18n(ja): fix false-friend mistranslations of mirror and local

[yahonda]

What is changed, added or deleted? (Required)

Two English words were mistranslated using their wrong homonym sense in 5 sites across 5 files:

• "mirror" (the TiUP package-repository concept) was rendered as 鏡 (the literal glass object), contradicting 190 other corpus sites that correctly use ミラー, and contradicting each affected file's own frontmatter summary (both already said ミラー). Fixed in tiup/tiup-command-mirror.md and tiup/tiup-command-mirror-sign.md, also restoring a dropped は particle in the former.
• "local" was rendered as 地元 ("hometown/native place") in three unrelated contexts:
  • grafana-tikv-dashboard.md: a TiKV Grafana panel heading "Local Reader," contradicting the correct ローカルリーダー used in the very next line of body text.
  • tidb-lightning/tidb-lightning-overview.md: a data-source list item "Local" (as in local filesystem).
  • tiup/tiup-component-cluster-template.md: the literal --local CLI flag name, which had also lost its double-dash syntax and picked up a stray leading dash (- 地元 instead of --local). Restored to the literal English flag name, matching its sibling --full/--multi-dc headings in the same file.

Also restores two untranslated cloud-provider proper nouns to English, matching the overwhelming corpus convention (Amazon S3: 484 occurrences, Google Cloud Storage: 82) and each file's own body text immediately below the affected heading, which already correctly kept them in English:

• tidb-lightning/tidb-lightning-overview.md: a data-source list item (アマゾンS3→Amazon S3, Googleクラウドストレージ→Google Cloud Storage).
• tidb-cloud/serverless-export.md: two section headings, same two terms.

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

N/A

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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