fix(v2-inspector): use API base URL for PPTX preview fetch

PptxPreview was calling `/api/uploads/.../preview-pptx-html` as a
relative URL — that hits the SPA host (app-dev.commonly.me), which
returns the index.html shell for any unknown path. The iframe srcdoc
got 830 bytes of HTML stub instead of the 17KB OfficeCli render, and
the inspector showed a black box.

Prepend `getApiBaseUrl()` (already used by signedAttachmentUrl helper
and the api hook) so the fetch hits the actual backend host. Verified
end-to-end: backend route returns 200 + 17.7KB text/html with embedded
slide canvas; the only thing missing was the host.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lilyshen0722

frontend/src/v2/components/V2PodInspector.tsx

@@ -8,6 +8,7 @@ import { UseV2PodsResult } from '../hooks/useV2Pods';
 import { useV2Api } from '../hooks/useV2Api';
 import { useAuth } from '../../context/AuthContext';
 import { getSignedAttachmentUrl } from '../../utils/signedAttachmentUrl';
+import getApiBaseUrl from '../../utils/apiBaseUrl';
 import type { InspectorView } from './V2Layout';
 
 interface V2PodInspectorProps {
@@ -612,7 +613,7 @@ const PptxPreview: React.FC<{ artifact: PreviewArtifact }> = ({ artifact }) => {
     setBusy(true); setError(null); setHtml(null);
     (async () => {
       try {
-        const r = await fetch(`/api/uploads/${encodeURIComponent(artifact.fileName!)}/preview-pptx-html`);
+        const r = await fetch(`${getApiBaseUrl()}/api/uploads/${encodeURIComponent(artifact.fileName!)}/preview-pptx-html`);
         if (cancelled) return;
         if (!r.ok) {
           // Surface a useful error from the JSON body if available