client/web: relax CSP restrictions for manage client

Don't return CSP headers in dev mode, since that includes a bunch of
extra things like the vite server.

Allow images from any source, which is needed to load user profile
images.

Allow 'unsafe-inline' for various inline scripts and style react uses.
We can eliminate this by using CSP nonce or hash values, but we'll need
to look into the best way to handle that. There appear to be several
react plugins for this, but I haven't evaluated any of them.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

client/web/web.go

@@ -220,10 +220,13 @@ func (s *Server) serve(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
+		if !s.devMode {
 			w.Header().Set("X-Frame-Options", "DENY")
-		w.Header().Set("Content-Security-Policy", "default-src 'self'")
+			// TODO: use CSP nonce or hash to eliminate need for unsafe-inline
+			w.Header().Set("Content-Security-Policy", "default-src 'self' 'unsafe-inline'; img-src * data:")
 			w.Header().Set("Cross-Origin-Resource-Policy", "same-origin")
 		}
+	}
 
 	if strings.HasPrefix(r.URL.Path, "/api/") {
 		switch {