fix(upload): raise body-size limit to 512 MiB + surface upload errors

Artifact uploads (PLC .projectarchive, firmware images, mobile packages) larger
than axum's 2 MiB default request-body limit were rejected, and the wizard
swallowed the error (`let _ = upload_target_artifact`), so the target was created
with no artifact — every scan then showed "blocked, no artifact provided".

- agent: DefaultBodyLimit::max(512 MiB) on the API router (multipart upload route).
- dashboard: DefaultBodyLimit::max(512 MiB) on the axum app serving the upload
  server function (the browser→dashboard hop).
- wizard: surface upload failures in the error banner instead of ignoring them.

Found live: a 9.9 MB CODESYS .projectarchive failed to attach (a 1.7 KB .st worked).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compliance-agent/src/api/server.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use axum::extract::Request;
+use axum::extract::{DefaultBodyLimit, Request};
 use axum::http::HeaderValue;
 use axum::middleware::Next;
 use axum::response::Response;
@@ -74,6 +74,9 @@ pub async fn start_api_server(agent: ComplianceAgent, port: u16) -> Result<(), A
 
     let mut app = routes::build_router()
         .merge(admin_router)
+        // Allow large artifact uploads (PLC .projectarchive, firmware images,
+        // mobile packages) — axum's default request-body limit is only 2 MiB.
+        .layer(DefaultBodyLimit::max(512 * 1024 * 1024))
         .layer(Extension(Arc::new(agent.clone())))
         .layer(CorsLayer::permissive())
         .layer(TraceLayer::new_for_http())