feat: add Keycloak authentication for dashboard and API endpoints (#2)

Dashboard: OAuth2/OIDC login flow with PKCE, session-based auth middleware
protecting all server function endpoints, check-auth server function for
frontend auth state, login page gate in AppShell, user info in sidebar.

Agent API: JWT validation middleware using Keycloak JWKS endpoint,
conditionally enabled when KEYCLOAK_URL and KEYCLOAK_REALM are set.

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

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #2

compliance-dashboard/src/infrastructure/error.rs

@@ -24,3 +24,14 @@ impl From<DashboardError> for ServerFnError {
         ServerFnError::new(err.to_string())
     }
 }
+
+#[cfg(feature = "server")]
+impl axum::response::IntoResponse for DashboardError {
+    fn into_response(self) -> axum::response::Response {
+        (
+            axum::http::StatusCode::INTERNAL_SERVER_ERROR,
+            self.to_string(),
+        )
+            .into_response()
+    }
+}