router: normalize base to always end with trailing slash

[nothing-stops-this-train]

Summary

Hard-to-spot bug: when BITCART_ADMIN_ROOTPATH is set to /admin (no trailing slash), every nested admin URL (/admin/plugins/<x>, /admin/stores/<id>/<x>, etc.) renders Nuxt's "404 Not Found" page on the first paint, even though the route is correctly registered with Vue Router and the page renders fine server-side.

Single-segment URLs (/admin/, /admin/stores, /admin/plugins) survive because their corrupted initial path coincidentally resolves to the home route, which clears the sticky err state on the next Vue Router navigation. Multi-segment URLs never recover.

Root cause

.nuxt/utils.js's getLocation helper does base.slice(0, -1), with the comment "consideration is base is normalized with trailing slash". When base is /admin (no trailing slash), the slice corrupts it to /admi. The subsequent path.startsWith(slicedBase) matches the leading /admi of /admin/<x> but slices the path by 5 chars, producing strings like n/<x> — non-routes. router.resolve returns matched=0, Nuxt's client.js navigation hook sets app.context.error({statusCode: 404, message: "This page could not be found"}), and the err state never gets cleared.

Headless-Chromium reproducer (before patch)

Polling $nuxt.nuxt.err and $route.path from page load (cookies seeded so we're logged in):

/admin/                          → t=510ms  null|/|matched=1                ← OK
/admin/stores                    → t=323ms  null|/stores|matched=1          ← OK
/admin/invoices                  → t=363ms  null|/invoices|matched=1        ← OK
/admin/plugins                   → t=301ms  null|/plugins|matched=1         ← OK
/admin/plugins/liquidityhelper   → t=227ms  null|/|matched=0                ← initial: no match
                                   t=305ms  404|/|matched=0                 ← 404 set
                                   t=365ms  404|/plugins/liquidityhelper|matched=1   ← route resolves but err is sticky

After patch, the same probe shows:

/admin/plugins/liquidityhelper   → t=487ms  null|/plugins/liquidityhelper|matched=1   ← OK from the start

Patch

Normalize config.ROOTPATH to always end with / at the router-construction point. Three reasons to do it here (rather than at the env-var parse site or in publicRuntimeConfig):

1. Doesn't affect other consumers of config.ROOTPATH — URL-construction code like ${config.ROOTPATH}/api/foo continues to see the unmodified value (e.g. /admin), so it doesn't accidentally produce double slashes like /admin//api/foo.
2. The bug is specifically about Vue Router's base. The fix lives where the value is consumed.
3. Single-line, well-localized, easy to revert if Nuxt's own getLocation is ever fixed upstream.

Normalization table

input	output
/admin	/admin/
/admin/	/admin/
/	/
""	/
undefined	/

Test plan

• Fresh URL-bar navigation to /admin/plugins/<any-plugin> no longer renders "404 Not Found".
• Existing admin routes (/admin/, /admin/stores, etc.) still load correctly.
• SPA navigation (clicking sidebar items) still works.
• Hard reload (Cmd-Shift-R) of nested URLs still works.
• On a root-path install (BITCART_ADMIN_ROOTPATH unset → "/"), behavior is unchanged — base stays /.

Note re: PR #1

This supersedes the diagnostic in PR #1. That PR addressed a symptom (async chunk 404 from missing /admin/ prefix on publicPath); this PR addresses the underlying cause of the visible "404 Not Found" page. The publicPath issue still happens but turns out to be cosmetic — it produces a single noisy console message for the workbox chunk, and doesn't break route resolution.