UI: activity client links, online badges, ended-leases toggle, All activity

- Activity: a client's MAC address opens its leases; "Only this" filters
  the list to that client.
- Reservations: Online is a green online / grey offline badge.
- A subnet's leases: "Include ended leases" is a checkbox.
- Dashboard: "All activity" is a button beside the heading.

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

internal/web/static/assets/pages/dashboard.js

@@ -50,7 +50,7 @@ export async function render(el) {
         problemsList(st.problems),
         st.ping_problem ? h("div", { class: "msg warn" }, st.ping_problem) : null) : null,
       events.length ? h("div", { class: "panel" },
-        h("h2", null, "Latest DHCP messages ", h("a", { href: "#/activity", class: "small" }, "all")),
+        h("div", { class: "panel-head" }, h("h2", null, "Latest DHCP messages"), h("a", { class: "button small", href: "#/activity" }, "All activity")),
         table([
           { label: "When", render: (e) => time(e.time) },
           { label: "Interface", render: (e) => e.interface },