Add Roles/MQTT-settings/Backup pages, AdminLTE demo shells, and a separate mobile design system

Backend: real endpoints for role-permission overview, MQTT broker settings
(with live reachability check), and SQLite backup download/restore (new
system.manage permission, super-admin only; restore validates the SQLite
file header and keeps a pre-restore safety copy).

Desktop frontend: Roles (read-only permissions view), MQTT Settings,
Backup & Restore wired to those endpoints; plus static AdminLTE-matching
shells for Profile, Settings (embeds the real 2FA flow in its Security
tab), Invoice, Calendar (FullCalendar), Chat, File Manager, and Projects.

Mobile frontend: new frontend/src/styles/mobile.css, a from-scratch design
system (purple theme, cards, slide-out sidebar, bottom tab bar) scoped
entirely under a .mob-app root and mob- prefixed classes so it can never
collide with the desktop Bootstrap/AdminLTE styling. Rebuilt TopBar/
BottomNav, added a new SlideOutNav, and reskinned the Dashboard and Users
pages to it with real data.

backend/app/Http/Controllers/Api/BackupController.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Services\Audit\AuditLogger;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Validation\ValidationException;
+
+class BackupController extends Controller
+{
+    private const SQLITE_HEADER = "SQLite format 3\000";
+
+    public function download(Request $request, AuditLogger $audit)
+    {
+        abort_unless($request->user()->can('system.manage'), 403);
+
+        $path = config('database.connections.sqlite.database');
+        abort_unless($path && is_file($path), 404, 'No database file to back up.');
+
+        $audit->log($request->user(), 'system.backup_downloaded');
+
+        return response()->download($path, 'backup-'.now()->format('Y-m-d-His').'.sqlite');
+    }
+
+    public function restore(Request $request, AuditLogger $audit)
+    {
+        abort_unless($request->user()->can('system.manage'), 403);
+
+        $request->validate(['backup' => ['required', 'file']]);
+
+        $uploaded = $request->file('backup');
+        $header = file_get_contents($uploaded->getRealPath(), false, null, 0, 16);
+
+        if ($header !== self::SQLITE_HEADER) {
+            throw ValidationException::withMessages([
+                'backup' => 'That file is not a valid SQLite database.',
+            ]);
+        }
+
+        $path = config('database.connections.sqlite.database');
+
+        // Release Laravel's open handle on the current file before swapping it.
+        DB::disconnect();
+
+        $safetyCopy = dirname($path).'/pre-restore-'.now()->format('Y-m-d-His').'.sqlite';
+        if (is_file($path)) {
+            copy($path, $safetyCopy);
+        }
+
+        $uploaded->move(dirname($path), basename($path));
+
+        $audit->log($request->user(), 'system.backup_restored');
+
+        return response()->noContent();
+    }
+}