nginx: security headers on every response (closes #18)

nginx sent none of the security headers REPO_POLICIES.md requires.
security-headers.conf now sets all six with always, included at server
level and again in /assets/, whose own add_header would otherwise drop
them. nginx hides the copies netwatch-server sets, so /api/ and the
health check carry each header once. The content security policy
allows no inline script or style; the host row's status dot took its
grey from a style attribute, now a class. connect-src is * because
several probed hosts redirect to other hosts and the browser checks
every redirect against it. Referrer-Policy is no-referrer, as the
backend already sends.

Model: opus-5-5

src/main.js

@@ -716,7 +716,7 @@ function hostRowHTML(host, index, showPin = true) {
         ${pinBtn}
         <div class="w-[420px] flex-shrink-0 grid grid-cols-[minmax(0,1fr)_auto] items-center">
           <div class="flex items-center gap-2 min-w-[200px]">
-            <div class="w-3 h-3 rounded-full flex-shrink-0" style="background-color: ${latencyHex(null)}"></div>
+            <div class="w-3 h-3 rounded-full flex-shrink-0 bg-[#6b7280]"></div>
             <span class="font-medium text-white truncate">${host.name}</span>
           </div>
           <div class="latency-value text-4xl font-bold tabular-nums text-right mt-3" data-host="${index}">