i18n(ar): add ar translation

[neoOpus]

i18n(ar): add Arabic translation for the documentation site

This PR adds the i18n/ar/ translation tree for Arabic (العربية, rtl)
to the ScriptCat documentation site. It depends on upstream-i18n-infra
(which registers the locale in docusaurus.config.js); merge that first.

What's included

• 44 docs mirroring docs/ 1:1 (strict parity, zero fallback):
  about, script installation, the full use/* guide set, the complete
  dev/* developer reference (including the Agent API section — all
  agent-* files match upstream's naming) and the use/policy/* legal
  pages, plus image assets.
• Changelogs: the full change/* set (version history index,
  beta-changelog, v0.16–v1.5) translated from the verified EN source.
• code.json: the exact upstream key set (268 keys) — homepage,
  GitHubStar, footer, navbar, search and install strings — translated.
• current.json: sidebar category labels (including the Agent and
  Service-Agreement sections).
• docusaurus-theme-classic/{navbar,footer}.json: theme strings.
• home.hero.en: the homepage hero tagline, translated.

Verified

• pnpm run check on the merged tree: strict i18n parity (ar: 44
  translated, 0 fallback), strict frontmatter, URL inventory — all green.
• Translated from the verified English source; structure, anchors and code
  blocks preserved; API names (GM_*, CAT.*, @grant, @crontab)
  verbatim.
• Glossary consistency per the project's conventions.

Nothing outside i18n/ar/ is touched.

[neoOpus]

Superseded by PR #87 (code.json) + PR #96 (dev docs).