panel: shared nav, account settings, backup page, connection settings

Phase 12 (UI/UX). The navigation bar now renders once from layout.html
instead of being copied into each content template, so it is present on
every authenticated page — including the domain page and its delete
confirmation, which had no links at all — and the current page is
highlighted via .Active rather than quietly dropping out of the list.

New /account page changes the administrator's username and/or password:
the current password is required and the attempt is throttled on the same
limiter as the login form, so this route cannot be used to brute-force
past that limit. A password change invalidates every other session while
keeping the one performing it; a rename carries that session over.

Backup and domain import move from a card in the middle of the domain
list to their own /backup page, one card each; the handlers themselves
are unchanged, only the page the import form renders its errors on.

The domain page gains a "Sending server settings" card (server, port,
encryption) so a client can be configured without reading the docs; 587
is listed only when SUBMISSION_ENABLE is true for this deployment, which
is a deploy-time flag the panel cannot verify at runtime.

Client-side (static/panel.js, no libraries): Copy buttons on the values
that get carried elsewhere (DKIM record, new application credentials,
server name), and the Addresses field is hidden while the address mode is
wildcard, where the server ignores it.

Verified in a container on the dev server: setup, login, every page's
nav and active item, domain and application creation, all account-form
paths including cross-session invalidation, import errors, full backup
download. gofmt/vet/test/docker build green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/admin.go

@@ -42,6 +42,29 @@ func (s *Store) CreateAdmin(username, passwordHash string) error {
 	return nil
 }
 
+// UpdateAdmin replaces the administrator's username and password hash. It
+// touches only the admin row (id = 1): panel credentials are unrelated to the
+// SASL logins applications authenticate with, which live in their own table.
+// ErrNoAdmin is returned if setup has not happened yet, so a change can never
+// silently create an account.
+func (s *Store) UpdateAdmin(username, passwordHash string) error {
+	res, err := s.db.Exec(
+		"UPDATE admin SET username = ?, password_hash = ? WHERE id = 1",
+		username, passwordHash,
+	)
+	if err != nil {
+		return fmt.Errorf("update admin: %w", err)
+	}
+	n, err := res.RowsAffected()
+	if err != nil {
+		return fmt.Errorf("update admin: %w", err)
+	}
+	if n == 0 {
+		return ErrNoAdmin
+	}
+	return nil
+}
+
 // GetAdmin returns the administrator account, or ErrNoAdmin if setup is pending.
 func (s *Store) GetAdmin() (Admin, error) {
 	var (

internal/store/admin_test.go

@@ -0,0 +1,45 @@
+package store
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestUpdateAdmin(t *testing.T) {
+	st := openTestStore(t)
+
+	if err := st.CreateAdmin("admin", "hash-one"); err != nil {
+		t.Fatalf("CreateAdmin: %v", err)
+	}
+	if err := st.UpdateAdmin("operator", "hash-two"); err != nil {
+		t.Fatalf("UpdateAdmin: %v", err)
+	}
+
+	a, err := st.GetAdmin()
+	if err != nil {
+		t.Fatalf("GetAdmin: %v", err)
+	}
+	if a.Username != "operator" || a.PasswordHash != "hash-two" {
+		t.Fatalf("unexpected admin after update: %+v", a)
+	}
+	if a.CreatedAt.IsZero() {
+		t.Fatal("update dropped created_at")
+	}
+}
+
+// An update before setup must not create the account: only the one-time setup
+// flow may do that (spec 7.6.1).
+func TestUpdateAdminWithoutAdmin(t *testing.T) {
+	st := openTestStore(t)
+
+	if err := st.UpdateAdmin("operator", "hash"); !errors.Is(err, ErrNoAdmin) {
+		t.Fatalf("UpdateAdmin without admin = %v, want ErrNoAdmin", err)
+	}
+	exists, err := st.AdminExists()
+	if err != nil {
+		t.Fatalf("AdminExists: %v", err)
+	}
+	if exists {
+		t.Fatal("UpdateAdmin created an administrator")
+	}
+}