feat(ui): surface a Wren AI Cloud upgrade entry point in the sidebar

[goldmedal]

What

Adds a Wren AI Cloud entry point to the sidebar, between the Learning section and
the Settings / Discord / GitHub footer links.

─────────────────────────────────────
 Learning                          ›
 ...
┌──────────────────────────────────┐
│  ✨ Upgrade to Wren AI Cloud     │
│  Team collaboration, hosted      │
│  deployment, and API access      │
└──────────────────────────────────┘
─────────────────────────────────────
 ⚙  Settings
 💬  Discord
 🐙  GitHub

Why

Self-hosted users have no in-product path to the hosted offering — the sidebar's
outbound links were limited to Discord and GitHub.

Details

• Single file touched: wren-ui/src/components/sidebar/index.tsx.
• The card is a styled(Link), matching the existing styled(Link) usage in
  SidebarTree.tsx, so the whole card is clickable rather than just the text.
• Colors come from the existing CSS variables (--geekblue-1/2/3/4/6, --gray-8);
  no new hard-coded values. Hover deepens the background and border together.
• Reuses the already-exported SparklesIcon from @/utils/icons; no new dependency.
• Follows the neighbouring links' PostHog convention
  (data-ph-capture-attribute-name="cta_go_to_cloud"), so its click-through can be
  measured alongside the Discord and GitHub links.
• Opens in a new tab with rel="noopener noreferrer".

Verification

• yarn check-types — pass
• next lint — ✔ No ESLint warnings or errors
• prettier --check on the touched file — pass
• Ran wren-ui locally (Node 20, SQLite) and confirmed the card renders in the
  sidebar on /modeling, /home, /knowledge/question-sql-pairs and
  /api-management/history.

🤖 Generated with Claude Code

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