fix(admin): match sidebar active state when a plugin page path is "/" - #3008

What does this PR do?

A plugin admin page declared with path: "/" never gets the active state in the sidebar. The link works and the page renders; only the highlight is missing, so the sidebar shows no selected item while you are on that page. This affects the shipped @emdash-cms/plugin-forms, whose first page is declared as { path: "/", label: "Forms" }.

Sidebar.tsx builds the target by concatenation (/plugins/${pluginId}${page.path}), so path: "/" yields /plugins/<id>/ with a trailing slash, while the router navigates to /plugins/<id>. isItemActive compared the two exactly, so the item could never match.

This normalizes the trailing slash on both sides before comparing, keeping "/" exact for the admin root. The fix is in the comparison rather than in the item builder, so it covers any nav item whose target ends in a slash, not just plugin pages.

Closes #2988

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm lint passes
• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• User-visible strings in the admin UI are wrapped for translation (if applicable) — no new strings
• I have added and reviewed the user-facing changeset (if this PR changes a published package)
• New features link to an approved Discussion — not a feature
• I have included screenshots below if this PR changes the UI

Environment note, so the three unchecked boxes above are not a mystery: I could not run the repo's own typecheck, lint and test locally. A scoped install (pnpm install --filter @emdash-cms/admin...) leaves the other workspace packages unbuilt, so pnpm typecheck in packages/admin fails on @emdash-cms/blocks, @emdash-cms/plugin-types and @emdash-cms/registry-client — pre-existing resolution errors unrelated to this change, and CONTRIBUTING says pnpm build is required first. The admin suite also runs in browser mode, which I did not set up. What I did run is below. CI is the source of truth here.

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: Claude Opus 5 (Claude Code). A human reviewed the diff and the measurements before it was opened.

Screenshots / test output

Both screenshots are from EmDash 0.36.0 running in astro dev, same sidebar, same session. The site has two plugins with admin pages: the shipped forms plugin (path: "/", hits the bug) and a local email provider that works around it with path: "/status".

Before — the forms plugin page is open (/plugins/emdash-forms), and no sidebar item is highlighted:

After — the same sidebar with a page whose path has a segment (/plugins/ses-email/status), where the highlight appears as expected. This is the state the patch gives back to path: "/" pages:

The rendered pixels do not change; what changes is which item receives the active state. The measurement behind the screenshots, reading data-active on the sidebar anchors in the page:

current path /_emdash/admin/plugins/emdash-forms
  /_emdash/admin/plugins/ses-email/status        act=
  /_emdash/admin/plugins/emdash-forms            act=          <- should be active
  /_emdash/admin/plugins/emdash-forms/submissions act=

current path /_emdash/admin/plugins/ses-email/status
  /_emdash/admin/plugins/ses-email/status        act=true
  /_emdash/admin/plugins/emdash-forms            act=
  /_emdash/admin/plugins/emdash-forms/submissions act=

Targeted verification of the new comparison, including the cases the existing behavior must keep:

PASS  isItemActive("/taxonomies/course?locale=de", "/taxonomies/course") -> true   existing: query is ignored
PASS  isItemActive("/plugins/emdash-forms/", "/plugins/emdash-forms") -> true   new: plugin page declared with path "/"
PASS  isItemActive("/plugins/emdash-forms/", "/plugins/emdash-forms/submissions") -> true   new: nested route stays active
PASS  isItemActive("/plugins/emdash-forms/", "/plugins/emdash-forms-extra") -> false   new: sibling sharing a prefix must not match
PASS  isItemActive("/", "/media") -> false   new: admin root stays exact
PASS  isItemActive("/", "/") -> true   new: admin root matches itself
PASS  isItemActive("/media", "/media") -> true   existing: plain route
PASS  isItemActive("/media", "/media/123") -> true   existing: nested under a plain route
PASS  isItemActive("/media", "/mediation") -> false   existing: prefix collision

9/9 passed

Four of these are added to the existing isItemActive block in packages/admin/tests/components/Sidebar.test.tsx.