fix(control-plane): drop locale slug from URLs (localePrefix never)

[nicoloboschi]

What does this PR do?

Control Plane loses the selected bank on page refresh for non-default locales.

Root cause: routing used localePrefix: "as-needed", so non-English locales carry a locale prefix in the URL (e.g. /es/banks/hermes). bank-context.tsx reads usePathname() from next/navigation (which includes the prefix) and parses the bank id with a ^/banks/ anchored regex, so the match fails and currentBank stays null on load. English (the default locale) had no prefix, so the bug only affected the other 9 locales.

Fix: switch next-intl to localePrefix: "never". The active locale is resolved from the NEXT_LOCALE cookie (Accept-Language fallback) and next-intl rewrites internally to the [locale] segment, so the locale never appears in the URL. Every path stays clean (/banks/x), and the existing ^/banks/ parsing works for all languages.

Also removes the now-dead stripLocalePrefix() helper in middleware.ts (there is no prefix to strip anymore).

Supersedes #2070

#2070 patched the same symptom by un-anchoring the regex. This addresses it at the routing layer instead: the locale slug was unintentional for an authenticated admin UI, and removing it keeps the address bar locale-agnostic. With this change #2070 is no longer needed.

Related Issue

Fixes #2069

Trade-offs

• Language is now a per-user cookie preference, not URL-addressable (no shareable /es/... deep links). Irrelevant for an authed control plane with no SEO surface.
• [locale] route folders are unchanged; next-intl rewrites to them internally.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

How to Test

1. Open Control Plane, select a bank, refresh → bank stays selected.
2. Switch language via the picker → UI translates, URL stays /banks/x (no /es/ prefix), locale persists across refresh via cookie.

Test Results

• tsc --noEmit: clean
• eslint (changed files): clean
• vitest: 65/65 pass