fix(ai-assist): prevent HTML-as-JSON crash; add safeJsonFetch + JSON-only API errors

- Created lib/safeJsonFetch.ts helper to safely parse JSON responses
- Updated AIAssist.tsx to use safeJsonFetch with proper Content-Type header
- Improved app/api/ai-assist/route.ts with requestId tracking and safe body parsing
- All API responses now return JSON with proper error messages

lib/safeJsonFetch.ts

@@ -0,0 +1,73 @@
+export class NonJsonResponseError extends Error {
+    status: number;
+    contentType: string | null;
+    bodyPreview: string;
+
+    constructor(args: { status: number; contentType: string | null; bodyPreview: string }) {
+        super(`Expected JSON but received ${args.contentType ?? "unknown content-type"} (HTTP ${args.status})`);
+        this.name = "NonJsonResponseError";
+        this.status = args.status;
+        this.contentType = args.contentType;
+        this.bodyPreview = args.bodyPreview;
+    }
+}
+
+type SafeJsonFetchResult<T> =
+    | { ok: true; data: T }
+    | { ok: false; error: { message: string; status?: number; bodyPreview?: string } };
+
+export async function safeJsonFetch<T>(
+    url: string,
+    init?: RequestInit
+): Promise<SafeJsonFetchResult<T>> {
+    const res = await fetch(url, init);
+
+    const contentType = res.headers.get("content-type");
+    const text = await res.text();
+
+    // HTTP error — return readable details (don't JSON.parse blindly)
+    if (!res.ok) {
+        // Try JSON first if it looks like JSON
+        if (contentType?.includes("application/json")) {
+            try {
+                const parsed = JSON.parse(text);
+                return { ok: false, error: { message: parsed?.error ?? "Request failed", status: res.status } };
+            } catch {
+                // fall through to generic
+            }
+        }
+        return {
+            ok: false,
+            error: {
+                message: `Request failed (HTTP ${res.status})`,
+                status: res.status,
+                bodyPreview: text.slice(0, 300),
+            },
+        };
+    }
+
+    // Success but not JSON => this is exactly the "Unexpected token <" case
+    if (!contentType?.includes("application/json")) {
+        return {
+            ok: false,
+            error: {
+                message: `Server returned non-JSON (content-type: ${contentType ?? "unknown"})`,
+                status: res.status,
+                bodyPreview: text.slice(0, 300),
+            },
+        };
+    }
+
+    try {
+        return { ok: true, data: JSON.parse(text) as T };
+    } catch {
+        return {
+            ok: false,
+            error: {
+                message: "Server returned invalid JSON",
+                status: res.status,
+                bodyPreview: text.slice(0, 300),
+            },
+        };
+    }
+}