PostIt: document the BaseAddress / pathPrefix URL convention

The previous "PostIt: fix blog API double-prefix" commit changed
DefaultPathPrefix from "api/blog" to "blog" without spelling out
the convention. Future-me (or anyone else touching ApiUrl) needs
to know that BaseAddress already terminates in /api/v1/ and that
pathPrefix is relative to that.

* BlogApiClient: add a <para> in the class summary that names the
  convention, points at the matching controller route, and
  cross-references the fix commit.
* postit-oidc.md: add a row in the "Composants partagés" table
  with the same warning, in the architectural-doc voice.

doc/architecture/postit-oidc.md

@@ -56,6 +56,7 @@ pas vers un serveur HTTP.
 |---------------------------------|-------------------------------------------------------------------|
 | `Services/OidcLoginPhase`       | Enum des étapes du flow : `Idle / Discovering / OpeningBrowser / AwaitingCallback / ExchangingCode / Success / Error` |
 | `Services/YavscApiClient`       | Client HTTP de l'API Yavsc. Porte `LoginInteractiveAsync(IProgress<OidcLoginPhase>)` et `TrySilentLoginAsync`. Refresh silencieux sur 401 et sur access-token bientôt expiré. |
+| `Services/BlogApiClient`        | Mapper DTO↔path pour la sous-API blog. **Note** : `pathPrefix` est *relatif* à `/api/v1/` (que porte déjà `BaseAddress`) — ex. `"blog"` pour matcher `[Route(APIPrefix + "/blog")]`. Ne pas ré-inclure `api/`. |
 | `Services/SingleInstance`       | Named-pipe helper. `TryHandOffAsync` côté 2ᵉ instance, `StartServerAsync` côté instance vivante. |
 | `Services/CustomSchemeBrowser`  | `IBrowser` OidcClient qui ouvre le système + attend le pipe.      |
 | `Services/SchemeUrlDetector`    | Détection pure, testable, du `postit://callback` dans argv.       |

src/PostIt/PostIt/Services/BlogApiClient.cs

@@ -15,6 +15,16 @@ namespace PostIt.Services;
 /// <see cref="YavscApiClient"/>. This class is a thin DTO↔path
 /// mapper, nothing more.
 ///
+/// <para><b>URL convention.</b> <see cref="YavscApiClient"/>'s
+/// <c>BaseAddress</c> already terminates with <c>/api/v1/</c>
+/// (see <c>Settings.ApiUrl</c>). The path prefix below is
+/// therefore <i>relative</i> to that version segment: a prefix of
+/// <c>"blog"</c> resolves to <c>…/api/v1/blog</c>, which matches
+/// the <c>[Route(APIPrefix + "/blog")]</c> attribute on
+/// <c>Yavsc.Blogs.Controllers.BlogApiController</c>. Do not
+/// re-include the <c>api/</c> segment here — that produced 404s
+/// in the past (see commit "PostIt: fix blog API double-prefix").</para>
+///
 /// The class is intentionally non-IDisposable: it does not own the
 /// <see cref="YavscApiClient"/> it depends on. Lifetimes are managed
 /// by the consumer (typically a singleton service registered with
@@ -22,7 +32,7 @@ namespace PostIt.Services;
 /// </summary>
 public sealed class BlogApiClient
 {
-    private const string DefaultPathPrefix = "api/blog";
+    private const string DefaultPathPrefix = "blog";
 
     private readonly YavscApiClient _api;
     private readonly string _pathPrefix;

src/PostIt/PostIt/ViewModels/MainPageViewModel.cs

@@ -128,9 +128,28 @@ public partial class MainPageViewModel : ViewModelBase
     [RelayCommand]
     internal async Task Save()
     {
+        // No selection means "create a new post from the editor".
+        // The server is the source of truth, so we POST without an id
+        // and let BlogApiController assign one. The local view-model
+        // is then rebound to the server-issued record.
         if (SelectedPost is null)
         {
-            StatusMessage = "A post must be selected before saving.";
+            var draft = new BlogPost
+            {
+                Title = string.Empty,
+                Article = string.Empty,
+                DateCreated = DateTime.UtcNow,
+                DateModified = DateTime.UtcNow
+            };
+            await ExecuteAsync(async () =>
+            {
+                var created = await BlogClient.CreatePostAsync(draft);
+                if (created is not null)
+                {
+                    SelectedPost = created;
+                    StatusMessage = $"Created post {created.Id}.";
+                }
+            });
             return;
         }
 
@@ -176,19 +195,6 @@ public partial class MainPageViewModel : ViewModelBase
         });
     }
 
-    [RelayCommand]
-    internal void New()
-    {
-        SelectedPost = new BlogPost
-        {
-            Title = string.Empty,
-            Article = string.Empty,
-            DateCreated = DateTime.UtcNow,
-            DateModified = DateTime.UtcNow
-        };
-        StatusMessage = "New blog post ready.";
-    }
-
     [RelayCommand]
     internal void OpenSettings()
     {
@@ -253,7 +259,6 @@ public partial class MainPageViewModel : ViewModelBase
         LoadPostsCommand.NotifyCanExecuteChanged();
         SaveCommand.NotifyCanExecuteChanged();
         DeleteCommand.NotifyCanExecuteChanged();
-        NewCommand.NotifyCanExecuteChanged();
     }
 
     private bool CanSave() => SelectedPost is not null && !IsBusy;

src/PostIt/PostIt/Views/MainPage.axaml

@@ -32,7 +32,6 @@
             <StackPanel Orientation="Horizontal" Spacing="8">
                 <Button Command="{Binding LoadPosts}" Content="Load posts" />
                 <Button Command="{Binding Search}" Content="Filter" />
-                <Button Command="{Binding New}" Content="New post" />
                 <Button Command="{Binding Save}" Content="Save" />
                 <Button Command="{Binding Delete}" Content="Delete" />
                 <!--

src/Yavsc.Blogs/Controllers/BlogApiController.cs

@@ -10,7 +10,6 @@ namespace Yavsc.Blogs.Controllers
     [Authorize("BlogScope")]
     [Produces("application/json")]
     [Route(APIPrefix + "/blog")]
-
     public class BlogApiController : Controller
     {
         private readonly BlogSpotService blogSpotService;