fix: add response.ok checks to all frontend fetch calls

[gordonmurray]

All fetch() calls in app.js call response.json() without first checking response.ok. If the backend returns a 4xx or 5xx error, the error JSON is silently parsed as valid data, which can cause confusing behavior in the UI.

Example (current):

const response = await fetch(`/datasets/${name}/rows?...`);
const data = await response.json();
// data might be an error object, but we treat it as row data

Proposed fix: Add a guard before each .json() call:

const response = await fetch(`/datasets/${name}/rows?...`);
if (!response.ok) {
    throw new Error(`API error: ${response.status} ${response.statusText}`);
}
const data = await response.json();

Applies to every fetch() call in app.js.