Since the record fence, vault previews load through object URLs: the
detail sheet frames a decrypted PDF from a blob: URL and thumbnails
render from blob: images. The app CSP carried no frame-src, so the PDF
iframe fell through to default-src 'self' and was blocked outright,
while img-src 'self' data: silently degraded every thumbnail to its
fallback icon.
Add frame-src 'self' blob: and extend img-src with blob: in both the
dev and prod policies. This admits exactly the app's own documents: a
blob: URL is mintable only by same-origin script, script execution
stays nonce-bound in production, and object-src 'none', frame-ancestors
'none' and default-src 'self' are untouched. The vault serve route
keeps its stricter document CSP. A unit test pins the directives so a
future CSP edit cannot silently regress the previews.
Refs #787
