feat(web): route unauthenticated root to /welcome, deep links to login

An unauthenticated visit to / now lands on the public /welcome page
instead of bouncing straight to Dex. Deeper guarded paths still redirect
to /auth/login so the post-login round-trip returns the visitor to the
page they asked for. isPublicPath runs first, so no redirect loop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/oidc/oidc_test.go

@@ -248,9 +248,15 @@ func TestMiddlewareRedirectsUnauthenticated(t *testing.T) {
 		w.WriteHeader(http.StatusOK)
 	}))
 
+	// The bare root sends an unauthenticated visitor to the public landing page.
 	rec := httptest.NewRecorder()
 	guarded.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+	require.Equal(t, http.StatusFound, rec.Code)
+	require.Equal(t, "/welcome", rec.Header().Get("Location"))
 
+	// A deeper guarded path goes to login so the post-login round-trip returns there.
+	rec = httptest.NewRecorder()
+	guarded.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/v/some-id", nil))
 	require.Equal(t, http.StatusFound, rec.Code)
 	require.Equal(t, "/auth/login", rec.Header().Get("Location"))
 }