fix: default drafts/published tab so the empty-state create button isn't hidden

[saqibmanan]

What

navigationTab (the drafts/published filter on the Datasets, UseCases, and Collaboratives dashboard pages) started as null on first paint, because useQueryState('tab', parseAsString) had no default. Each page's empty-state branch only renders the "Add New X" button when navigationTab === 'drafts' — so for any account whose default-filtered list has zero items, the button (and the visually-selected tab) just didn't render on first load. A useEffect already tried to self-correct by calling setNavigationTab('drafts') when null, but effects run after first paint, so it's a race: the URL-update + re-render round-trip usually wins on a fast connection, and loses under real load or a slower query.

Root cause trail

Found while investigating CivicDataSpace-test#103 (test_prv_006/007 flaky under concurrent CI load). That specific case turned out to be a real, separately-fixed backend bug (DataSpaceBackend#199/PR#203) — but chasing the same symptom class turned up test_prv_011_org_create_collaborative timing out the same way, deterministically, every run, concurrent or not, for an account with zero existing collaboratives. Not flakiness — a real zero-item edge case nobody hit before because the usual test accounts have hundreds of existing records.

dataset/ and usecases/ pages share the byte-identical pattern (confirmed by inspection), so any account whose draft-filtered list happens to be empty hits the same wall there too.

Fix

useQueryState('tab', parseAsString.withDefault('drafts')) — same pattern already used in charts/page.tsx and the dataset access page. navigationTab is synchronously 'drafts' on first render, no race. Removed the now-dead null-check in each page's useEffect (state can no longer be null); kept the refetch-on-tab-change behavior.

Verification

• tsc --noEmit: clean on all three files (10 pre-existing errors elsewhere, in charts/components/ChartsImage.tsx, untouched by this change).
• eslint: clean on all three files.
• Not yet verified against a live running instance — that needs local Keycloak/NextAuth setup, out of scope for reaching this fix. Next step: confirm test_prv_002/003/011 go green against a dev deploy of this branch.