Hand the CI runner placeholder Supabase env vars

lib/supabase.ts calls createClient at module load, and Supabase's URL
validator throws when VITE_SUPABASE_URL is undefined — which it is on
the Gitea runner. AccountDetail and Transactions tests transitively
import TransactionForm → lib/supabase and trip on it; tests that mock
lib/supabase directly slip past. Set explicit placeholder values for
the Vitest step so the import succeeds; real auth calls are stubbed
inside the tests, so the placeholders are never sent over the wire.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -29,4 +29,11 @@ jobs:
         run: pnpm test:backend
 
       - name: Frontend tests (Vitest)
+        # Supabase's createClient validates the URL at module load, so the
+        # frontend tests need *some* value to import lib/supabase without
+        # throwing. Real auth calls are stubbed in the tests, so these are
+        # purely structural placeholders — never used over the wire.
+        env:
+          VITE_SUPABASE_URL: http://localhost:54321
+          VITE_SUPABASE_ANON_KEY: placeholder-anon-key-for-tests-only
         run: pnpm test:frontend