feat: separate create/edit form pages, Playwright test infrastructure, /auth/token endpoint

Move all CRUD create/edit forms from inline on list pages to dedicated form pages
with back-button navigation and post-save redirect. Add Playwright Docker container
(browser server on port 3000) with 25 passing E2E tests covering login, navigation,
and site CRUD workflows. Add POST /auth/token endpoint for clean JWT retrieval.

src/ScadaLink.CentralUI/Components/Pages/Design/DbConnectionForm.razor

@@ -0,0 +1,120 @@
+@page "/design/db-connections/create"
+@page "/design/db-connections/{Id:int}/edit"
+@using ScadaLink.Security
+@using ScadaLink.Commons.Entities.ExternalSystems
+@using ScadaLink.Commons.Interfaces.Repositories
+@attribute [Authorize(Policy = AuthorizationPolicies.RequireDesign)]
+@inject IExternalSystemRepository ExternalSystemRepository
+@inject NavigationManager NavigationManager
+
+<div class="container-fluid mt-3">
+    <button class="btn btn-link text-decoration-none ps-0 mb-2" @onclick="GoBack">&larr; Back</button>
+
+    <h4 class="mb-3">@(Id.HasValue ? "Edit Database Connection" : "Add Database Connection")</h4>
+
+    @if (_loading)
+    {
+        <LoadingSpinner IsLoading="true" />
+    }
+    else
+    {
+        <div class="card">
+            <div class="card-body">
+                <div class="mb-3">
+                    <label class="form-label">Name</label>
+                    <input type="text" class="form-control" @bind="_name" />
+                </div>
+                <div class="mb-3">
+                    <label class="form-label">Connection String</label>
+                    <input type="text" class="form-control" @bind="_connectionString" />
+                </div>
+                <div class="mb-3">
+                    <label class="form-label">Max Retries</label>
+                    <input type="number" class="form-control" @bind="_maxRetries" min="0" />
+                </div>
+                <div class="mb-3">
+                    <label class="form-label">Retry Delay (seconds)</label>
+                    <input type="number" class="form-control" @bind="_retryDelaySeconds" min="0" />
+                </div>
+
+                @if (_formError != null)
+                {
+                    <div class="text-danger small mb-2">@_formError</div>
+                }
+
+                <div class="d-flex gap-2">
+                    <button class="btn btn-success" @onclick="Save">Save</button>
+                    <button class="btn btn-outline-secondary" @onclick="GoBack">Cancel</button>
+                </div>
+            </div>
+        </div>
+    }
+</div>
+
+@code {
+    [Parameter] public int? Id { get; set; }
+
+    private bool _loading = true;
+    private string _name = "", _connectionString = "";
+    private int _maxRetries = 3;
+    private int _retryDelaySeconds = 5;
+    private string? _formError;
+
+    private DatabaseConnectionDefinition? _existing;
+
+    protected override async Task OnInitializedAsync()
+    {
+        if (Id.HasValue)
+        {
+            try
+            {
+                _existing = await ExternalSystemRepository.GetDatabaseConnectionByIdAsync(Id.Value);
+                if (_existing != null)
+                {
+                    _name = _existing.Name;
+                    _connectionString = _existing.ConnectionString;
+                    _maxRetries = _existing.MaxRetries;
+                    _retryDelaySeconds = (int)_existing.RetryDelay.TotalSeconds;
+                }
+            }
+            catch (Exception ex) { _formError = ex.Message; }
+        }
+        _loading = false;
+    }
+
+    private async Task Save()
+    {
+        _formError = null;
+        if (string.IsNullOrWhiteSpace(_name) || string.IsNullOrWhiteSpace(_connectionString))
+        {
+            _formError = "Name and connection string required.";
+            return;
+        }
+
+        try
+        {
+            if (_existing != null)
+            {
+                _existing.Name = _name.Trim();
+                _existing.ConnectionString = _connectionString.Trim();
+                _existing.MaxRetries = _maxRetries;
+                _existing.RetryDelay = TimeSpan.FromSeconds(_retryDelaySeconds);
+                await ExternalSystemRepository.UpdateDatabaseConnectionAsync(_existing);
+            }
+            else
+            {
+                var dc = new DatabaseConnectionDefinition(_name.Trim(), _connectionString.Trim())
+                {
+                    MaxRetries = _maxRetries,
+                    RetryDelay = TimeSpan.FromSeconds(_retryDelaySeconds)
+                };
+                await ExternalSystemRepository.AddDatabaseConnectionAsync(dc);
+            }
+            await ExternalSystemRepository.SaveChangesAsync();
+            NavigationManager.NavigateTo("/design/external-systems");
+        }
+        catch (Exception ex) { _formError = ex.Message; }
+    }
+
+    private void GoBack() => NavigationManager.NavigateTo("/design/external-systems");
+}