Rework client OIDC env vars: rename to VITE_OIDC_*, add committed .env

Renames VITE_AUTH_* to VITE_OIDC_* to match the stack convention.
Adds a dedicated VITE_OIDC_POST_LOGOUT_REDIRECT_URI instead of deriving
it from the redirect URI via string replace. Switches from Dockerfile
ARG/ENV to a committed src/Budget.Client/.env so Vite picks up
production values at build time without needing build-arg overrides.
.env.local is gitignored for localhost dev overrides.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -2,4 +2,7 @@ bin/
 obj/
 /packages/
 riderModule.iml
-/_ReSharper.Caches/
+/_ReSharper.Caches/
+
+# Local dev overrides (contain localhost URLs, not secrets)
+src/Budget.Client/.env.local