fix(ci): make e2e_ui_testing actually test the freshly built UI bundle

The Build UI from source step used:

    cp -r out/ ../../litellm/proxy/_experimental/out/

GNU cp (CircleCI's Ubuntu image, coreutils 8.32) interprets this as
copy the source directory as a CHILD of the destination when the
destination already exists — so the command silently created
litellm/proxy/_experimental/out/out/ instead of replacing the served
bundle at litellm/proxy/_experimental/out/*.

The proxy continued serving whatever bundle was checked in, so every
e2e_ui_testing run between this job's introduction (d09d98a70a,
2026-04-08) and the bundle-rebuild commit (de790fd273, 2026-04-18) was
effectively testing a STALE bundle — not the fresh build. That is why
the double-prefix regression (NEXT_PUBLIC_BASE_URL="ui/" combined with
networking.tsx reading the env var) was never caught in CI even though
the source contained the trigger the whole time: the bundle the proxy
served never picked up the source change.

Replace cp -r with rm + mv so the destination is cleanly swapped.

Verified end-to-end on an Ubuntu 22.04 / GNU coreutils 8.32 container:
- Before fix: fresh build has 9 "ui/" literals in chunks; after cp,
  _experimental/out/*  still has 0 (stale); _experimental/out/out/ is a
  nested dir the proxy does not serve.
- After fix: _experimental/out/*  has 9 "ui/" literals — the proxy now
  serves the freshly built (broken, in this repro) bundle, so
  globalSetup fails at login and every spec is blocked. Removing the
  bug from .env.production and rebuilding brings the count back to 0
  and the suite passes.

No spec changes, no fixtures, no new infrastructure. The existing
Playwright suite already catches this class of regression via the
login flow in globalSetup; it just needs the CI to actually hand it
the freshly built bundle.

.circleci/config.yml

@@ -3051,10 +3051,19 @@ jobs:
             - ui/litellm-dashboard/node_modules
       - run:
           name: Build UI from source
+          # Prior version used `cp -r out/ ../../litellm/proxy/_experimental/out/`.
+          # GNU cp (used on CircleCI's Ubuntu image) interprets that as "copy the
+          # source directory as a child of the destination" when the destination
+          # already exists — silently creating `_experimental/out/out/` instead of
+          # replacing the served bundle. The proxy continued serving whatever was
+          # checked into `_experimental/out/*`, so this job was effectively testing
+          # the pre-build bundle on every run. Replace-and-move guarantees the
+          # freshly built bundle is what the proxy actually serves.
           command: |
             cd ui/litellm-dashboard
             npm run build
-            cp -r out/ ../../litellm/proxy/_experimental/out/
+            rm -rf ../../litellm/proxy/_experimental/out
+            mv out ../../litellm/proxy/_experimental/out
             # Restructure HTML so extensionless routes work (login.html -> login/index.html)
             find ../../litellm/proxy/_experimental/out -name '*.html' ! -name 'index.html' | while read -r f; do
               d="${f%.html}"; mkdir -p "$d"; mv "$f" "$d/index.html"