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

TODO.md

@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx sends the security headers `REPO_POLICIES.md` requires on
+  every response (issue #18), including errors, `/assets/` and what it passes on
+  from the backend, whose own copies it drops so each header goes out once. They
+  live in `security-headers.conf`, which `nginx.conf` includes. The content
+  security policy allows no inline script or style, so the status dot's grey in
+  `src/main.js` is now a class; `connect-src` is `*` because probed hosts
+  redirect to others, and the browser checks each redirect against it
 - 2026-09-29: nginx takes the client address from `X-Forwarded-For` only on
   requests from the reverse proxies named in the container's `TRUSTED_PROXIES`
   (issue #64), and by default from none, where it trusted every RFC1918 address