Add config-driven extension hook for sidebar

[msykes]

Contribution Type

• Recipe (/recipes)
• Schema (/schemas)
• Dashboard (/dashboards)
• Integration (/integrations)
• Skill (/skills)
• Repo improvement (docs, CI, templates)

What does this do?

Adds a small drop-in extension surface so dashboard add-ons can register a new route + sidebar entry without touching core files:

• extensions.config.ts: typed registry, empty by default
• components/Sidebar.tsx: splits nav into core + extensions + trailing, resolves icon keys via a registry; adds clock/folder/plug/sparkles
• lib/api.ts: exports apiFetch so extension pages can reuse the authenticated JSON fetch + error plumbing
• EXTENSIONS.md: convention doc (folder layout, auth, icon registry, sidecar vs in-tree REST routes)

After this change, future extensions touch only their own folder under app// and a single entry in extensions.config.ts.

Requirements

No new dependencies. No new external services. Compatible with the existing
Next.js App Router stack already in open-brain-dashboard-next. Does not
depend on any canonical skill or primitive — this is an in-tree convention
for the dashboard itself.

Checklist

• I've read CONTRIBUTING.md
• [N/A] My contribution has a README.md with prerequisites, step-by-step instructions, and expected outcome
• [N/A] My metadata.json has all required fields
• [N/A] If my contribution depends on a skill or primitive, I declared it in metadata.json and linked it in the README
• I tested this on my own Open Brain instance
• No credentials, API keys, or secrets are included

Note on the N/A items: This PR modifies an existing contribution
(dashboards/open-brain-dashboard-next) rather than adding a new one,
so the README / metadata.json checklist items don't apply in the usual
sense — the existing dashboard's README + metadata.json continue to
cover it. The new convention doc is EXTENSIONS.md inside the dashboard
folder, sibling to its existing README.

[github-actions]

Hey @msykes — welcome to Open Brain Source! 👋

Thanks for submitting your first PR. The automated review will run shortly and check things like metadata, folder structure, and README completeness. If anything needs fixing, the review comment will tell you exactly what.

Once the automated checks pass, a human admin will review for quality and clarity. Expect a response within a few days.

If you have questions, check out CONTRIBUTING.md or open an issue.

[alanshurafa]

Thanks for the contribution, and welcome. This is a clean extensibility hook — the config-driven approach (extensions.config.ts plus a Sidebar.tsx that reads it) lets dashboard extensions add a route and nav entry without touching core dashboard files, which is the right design for keeping the core stable. EXTENSIONS.md documents the pattern clearly.

The change is almost entirely additive (+181/-2). I reviewed it structurally rather than running the dashboard. Recommend maintainer review.

— Alan (community reviewer; non-binding)