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>
