Connections, "every packet" rules, and a packet tracer

- Connections: the kernel's tracked connections, read and closed over
  ctnetlink (internal/conntrack, raw netlink like the drop log), with
  their NAT and TCP state; closing one makes the rules check its next
  packet again (GET /connections, POST /connections/close; hc
  connections; Traffic → Connections).
- Rules can check every packet (every_packet, --every-packet): before
  established connections are accepted, so a drop or reject also stops
  connections already open. They can't accept.
- A packet tracer (internal/trace): a table of its own, inet
  housecarl-trace, marks the packets a filter picks, `nft monitor trace`
  reports every rule and policy they meet in every table, and the tracer
  names what accepted or dropped them, Housecarl's rules by name (POST,
  GET, DELETE /trace; hc trace; Traffic → Trace). One trace at a time,
  at most 200 packets and 20 a second; the table goes when it ends.

e2e: 157 checks, among them an upload a normal block doesn't stop and an
every-packet one does, and a trace naming the blocking rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/style.css

@@ -412,3 +412,10 @@ select.inline { width: auto; display: inline-block; }
 /* The rule dialog's optional parts: "+" buttons, and ✕ to close one. */
 .adders { display: flex; flex-wrap: wrap; gap: 6px; }
 button.close-section { padding: 0 6px; margin-left: 4px; font-size: 11px; line-height: 1.5; }
+/* Trace: one panel per packet, its steps below. */
+.trace-head { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; margin-bottom: 4px; }
+.trace-time { margin-left: auto; }
+table.trace-steps { width: 100%; margin-top: 6px; }
+table.trace-steps td { padding: 3px 8px; border-top: 1px solid var(--border); }
+tr.step-drop td { background: var(--del); }
+tr.step-accept td { background: var(--add); }