The catch-all /* -> /index.html redirect was serving HTML for /og-image.png,
so Signal's crawler got text/html instead of image/png. Added explicit rules
for static files (og-image.png, logo.png, robots.txt, manifest.webmanifest)
before the SPA fallback so they serve with correct content types.
Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>