fix(dsh-plugin): add PiP close button and sniff observation thumbnail media type

The Document PiP window had no close/return entry once popped out —
users could not dismiss it from inside the window. Add an onClosePip
callback and render a close button in the PiP header; clicking it
calls pipWindow.close() and the content returns to its carrier.

The observation thumbnail publisher hardcoded mediaType as "image/png"
regardless of the actual capture bytes. Reuse the existing
sniffImageMediaType helper (exported from image.ts) so the HTTP
thumbnail endpoint declares the correct Content-Type when Chromium
returns JPEG captures.

Closes #153 (Bug 2 fully; Bug 1 was fixed in #135, this covers the
remaining observation-thumbnail path).

Co-authored-by: Cursor <cursoragent@cursor.com>

packages/dsh-plugin-browserskill/src/client/observation-sidebar.tsx

@@ -149,6 +149,7 @@ export function ObservationSidebarTab({
       now={now}
       inPip={pipWindow !== null}
       onPopOut={pipWindow === null && pipSupported ? () => popOut() : undefined}
+      onClosePip={pipWindow !== null ? () => pipWindow.close() : undefined}
     />
   );