fix(ui): actionable 413/504/401 toasts + VPS nginx deploy template

Production QA reported "Submit failed 413" on /generation and plain
"Upload failed" (no detail) on resource upload. Root cause is the
outer reverse-proxy nginx on the VPS — still on default
`client_max_body_size 1m` — rejecting requests before they reach
Odoo. The inner docker-nginx and Odoo limits were already raised in
earlier commits; only the VPS proxy was left unpatched.

UI side
- Add `describeApiError(err, fallback, context)` helper in
  lib/api-client.ts. Maps common HTTP codes to human-readable,
  actionable toast descriptions (413 → "ask ops to raise nginx
  client_max_body_size", 504 → "server took too long, retry", 502
  → "Odoo is restarting", 401 → "sign in again", 4xx/5xx → server
  error body + status code).
- Use it in GenerationPage submit, ResourceManager upload,
  TeacherLibrary upload, CustomExamCreate save/publish. Replaces
  four slightly-different ad-hoc mappings with one source of truth.

Deploy side
- Add deploy/nginx-vps.conf.example: full TLS reverse-proxy template
  with the body/timeout knobs that must match Odoo's limit_request
  (128 MB) and limit_time_real (900 s).
- Add docs/DEPLOY_413_504_FIX.md: step-by-step remediation guide for
  the team lead covering the 3 layers that can block large bodies
  (outer nginx, Cloudflare, Odoo worker) and how to verify each.

Made-with: Cursor

frontend/src/lib/api-client.ts

@@ -40,6 +40,64 @@ export class ApiError extends Error {
   }
 }
 
+/**
+ * Turn an arbitrary thrown value from an API mutation into a human-readable,
+ * actionable toast description. Deployment QA kept reporting generic
+ * "Submit failed" / "Upload failed" toasts — this helper surfaces the HTTP
+ * status and, when we recognise the code, a hint about what to do next
+ * (payload too big → ask ops to raise nginx, timeout → retry, etc.).
+ *
+ * @param err     the error thrown by an `api.*` call (usually an ApiError)
+ * @param fallback text to show if we can't classify the error
+ * @param context short context word used in the 413/504 hints
+ *                ("file", "request", "payload" …)
+ */
+export function describeApiError(
+  err: unknown,
+  fallback = "Something went wrong",
+  context = "request",
+): string {
+  const e = err as { status?: number; message?: string; data?: unknown } | null;
+  const status = e?.status;
+  // Try to extract a server-side error body even if the generic ApiError
+  // message wasn't populated (nginx 413/504 respond with an HTML page so
+  // `data.error` is empty and `message` looks like "413 ").
+  const serverMsg =
+    e?.data && typeof e.data === "object" && "error" in (e.data as object)
+      ? String((e.data as { error: unknown }).error || "").trim()
+      : "";
+
+  if (status === 413) {
+    return `The ${context} is too large for the server to accept (HTTP 413). Try a smaller file / fewer tasks, or ask an admin to raise the nginx \`client_max_body_size\` and Odoo \`limit_request\`.`;
+  }
+  if (status === 504) {
+    return `The server took too long to respond (HTTP 504). The ${context} may still be processing — wait a minute and reload before retrying.`;
+  }
+  if (status === 502) {
+    return "The backend is unreachable (HTTP 502). Odoo may be restarting — try again in a moment.";
+  }
+  if (status === 401) {
+    return "Your session expired — please sign in again.";
+  }
+  if (status === 403) {
+    return serverMsg || "You don't have permission to do that (HTTP 403).";
+  }
+  if (status === 404) {
+    return serverMsg || "The requested item no longer exists (HTTP 404).";
+  }
+  if (status === 422 || status === 400) {
+    return serverMsg || `The server rejected the ${context} as invalid (HTTP ${status}).`;
+  }
+  if (status && status >= 500) {
+    return `${serverMsg || "The server returned an error"} (HTTP ${status}). Check the Odoo logs for a stack trace.`;
+  }
+
+  // Network failure / fetch aborted / CORS — ApiError not thrown at all.
+  if (!status && e?.message) return e.message;
+
+  return fallback;
+}
+
 const ACCESS_KEY = "encoach_token";
 const REFRESH_KEY = "encoach_refresh_token";
 const EXP_KEY = "encoach_token_exp";