Add authentication: admin/agent bearer tokens for the API, login for the dashboard

control-api: every route now carries a mandatory access level (admin / agent /
open) in a route table. All /api/v1/admin/* require the admin token; the
write calls of validator-agent and prober (self-check, events, results,
complete) require a separate static agent token; register, heartbeat and
fetching the assignment stay open. Tokens come from env vars, are compared in
constant time and never logged. An empty token leaves that level open with a
startup warning (backward compatible).

validator-agent / prober: apiclient sends the agent token only to control-api.

admin-dashboard: login/password (from env) with a stateless HMAC session
cookie, Origin-based CSRF check, per-IP brute-force throttle, HX-Redirect for
htmx polls, logout in the sidebar; the dashboard calls control-api with the
admin token. Login page layout fixed after review.

Also: env plumbing in docker-compose/rxprod-compose/systemd/config examples,
e2e script with token assertions, tests, docs (API, SETUP, USAGE, DASHBOARD,
README), plan and review under docs/changes/, bin/ rebuilt with new
SHA256SUMS.

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

internal/dashboard/static/dashboard.css

@@ -337,7 +337,7 @@ nav.nav-groups { display: flex; flex-direction: column; gap: 1px; }
 }
 .form-check { display: flex; align-items: center; gap: 6px; font-size: 13px; }
 
-input[type="text"], input[type="number"], input[type="search"], textarea, select {
+input[type="text"], input[type="password"], input[type="number"], input[type="search"], textarea, select {
   font-family: var(--font-ui);
   font-size: 13.5px;
   padding: 7px 10px;
@@ -544,3 +544,18 @@ tr.edit-row textarea { flex: 1 1 auto; }
    Height is set by JS (see templates/targets.html), computed from the
    field's own line-height, so it stays correct if font-size ever changes. */
 textarea.autosize { overflow-y: hidden; resize: none; }
+
+/* ---------- login page & sidebar user ---------- */
+.login-shell { min-height: 100vh; display: flex; align-items: center; justify-content: center; padding: 24px 16px; position: relative; }
+.login-card { width: 100%; max-width: 380px; }
+.login-card h1 { font-size: 18px; margin: 0 0 4px; }
+.login-card .panel-body { display: flex; flex-direction: column; gap: 14px; }
+/* .field is flex: 1 1 220px (for rows); in this column the basis would become a 220px height. */
+.login-card .field { flex: 0 0 auto; }
+.sidebar-user {
+  margin-top: auto; margin-bottom: 8px;
+  display: flex; align-items: center; justify-content: space-between; gap: 8px;
+  font-family: var(--font-display); font-size: 11px; color: var(--text-muted);
+}
+.sidebar-user-name { min-width: 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+.sidebar-user + .sidebar-foot { margin-top: 0; }