ci: fix test-storybook config-dir + add missing behandelportal a11y coverage

WP-67's monorepo split renamed .storybook to .storybook-ssp/
.storybook-behandelportal, but test-storybook still defaulted to the
(now nonexistent) plain .storybook dir -- it loads <config-dir>/main.js
for test-runner hooks even in --url mode, so every invocation failed
with "Could not load main.js in .storybook".

Also: test-storybook:ci only ever built+served+tested the ssp instance.
Since the split, behandelportal's stories (werkvoorraad, beoordeling,
besluit-form, ...) were never axe-tested in CI at all. Added the
:behandelportal siblings (mirroring the existing storybook/
build-storybook naming) and wired them into ci-local.sh's --full step
and the storybook-a11y GitHub Actions job.

Verified directly: ssp 62/180 stories green, behandelportal 45/112
green (including the new besluit-form story), full `ci-local.sh --full`
green end-to-end.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -168,6 +168,10 @@ jobs:
         if: needs.changes.outputs.frontend == 'true'
       - run: npm run test-storybook:ci
         if: needs.changes.outputs.frontend == 'true'
+      - run: npm run build-storybook:behandelportal
+        if: needs.changes.outputs.frontend == 'true'
+      - run: npm run test-storybook:ci:behandelportal
+        if: needs.changes.outputs.frontend == 'true'
 
   backend:
     needs: changes