Add roles and a user management tab for admins (#180, #179)

Until now every account could do the same, and managing accounts
meant running make-user.php on the server. Accounts now carry a role,
and admins get a tab in the editor to create, rename, re-role, reset
and delete accounts. Both issues share the same server files, so they
land in one commit.

#180 Roles:
- Every account is admin or user. A missing or unknown role counts as
  user, on the server and in the client.
- auth/session and auth/login return {user, role}.
- Auth::requireAdmin() answers everyone else with 403.
- make-user.php takes the role as a third argument. Without it an
  existing account keeps its role, so a password reset from the CLI
  never demotes an admin.
- Admins see and save other users' projects like their own. Saving
  keeps the original owner.

#179 User management:
- The account menu gets "User management" (admins only) and "Plugins"
  (everyone). Both open the same closable tab, accounts on top and
  plugins below. The fixed Plugins tab is gone.
- Per account: Rename, Role, Password (no old password needed) and
  Delete with a confirmation. "New user..." creates one.
- Each account lists its projects with an Open button.
- Delete is refused while the account still owns projects.
- Renaming moves the projects along and ends every session of the
  account. transfer takes the same lock as save.
- On their own row an admin can only set the password. The last admin
  cannot be demoted or deleted. The server enforces both.
- The account rules live in AccountRules.php and accountRules.ts as
  pure functions, so they are testable without a server.

Findings:
- At 1280 px the Delete button reached past the column edge and was
  not clickable. The name column is flexible now, and module 49 checks
  the layout.
- The server lets an admin delete other users' projects, but the
  editor has no button for it yet. Left open in the issue.

Tests:
- ./check.sh: 148 files, 2214 tests, exit 0 (was 147 / 2204).
- New tests/php/accounts.test.php, 51 checks, plain PHP. CI runs it
  as its own step in the gate job, the host has no PHP.
- New e2e module 49 with 18 checks. CI creates the e2e-admin account
  in serve.sh. Modules 14, 38, 46 and the scaffold follow the new tab.
- e2e 1-16,37,38,45,46,49: all passed, 581 checks.
- Counter-checks: owner on save, admin view, default role and self
  delete in PHP fail 7, 5, 5 and 1 checks. Unchecked requireAdmin,
  a wider column and the caller as owner each fail exactly their
  e2e check.
- Check sum 1314 -> 1333 (+1 in 38, +18 in 49), computed.

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

api/lib/AccountRules.php

@@ -0,0 +1,52 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * What an administrator may not do to an account (#179), as plain functions of the facts: each
+ * answers the refusal to send back, or null when the change may go ahead.
+ *
+ * An administrator's own name and role are off limits here – renaming yourself would end the
+ * session you are using, and demoting yourself would lock you out of the page you are on. Their
+ * own password may be set like anyone's.
+ */
+final class AccountRules
+{
+    public static function renameRefusal(string $actor, string $target): ?string
+    {
+        return strcasecmp($actor, $target) === 0 ? 'You cannot rename your own account' : null;
+    }
+
+    public static function roleRefusal(string $actor, string $target, string $currentRole, string $newRole, int $admins): ?string
+    {
+        if (strcasecmp($actor, $target) === 0 && $newRole !== $currentRole) {
+            return 'You cannot change your own role';
+        }
+        if ($currentRole === 'admin' && $newRole !== 'admin' && $admins <= 1) {
+            return 'The last administrator cannot be demoted';
+        }
+        return null;
+    }
+
+    public static function deleteRefusal(string $actor, string $target, string $role, int $admins): ?string
+    {
+        if (strcasecmp($actor, $target) === 0) {
+            return 'You cannot delete your own account';
+        }
+        if ($role === 'admin' && $admins <= 1) {
+            return 'The last administrator cannot be deleted';
+        }
+        return null;
+    }
+
+    /**
+     * An account with projects is not deleted: they would belong to nobody and vanish from every
+     * list. The administrator deletes them first – there is no way to hand them on yet.
+     */
+    public static function ownerRefusal(string $target, int $projects): ?string
+    {
+        if ($projects === 0) {
+            return null;
+        }
+        return "{$target} still owns {$projects} " . ($projects === 1 ? 'project' : 'projects');
+    }
+}