feat(postit): circles+ACL UI, blog fixture→SQLite, seed default user

Bundled end-of-branch commit on feat/postit-acl-members.

PostIt UI for circles + per-post ACL
- Reorganise PostIt.Tests into Auth/ and Blogs/ subfolders
  (Bearer/OIDC scope tests vs. blog API fakes live where they
  belong) and introduces PostItHeadlessCollection so the
  Avalonia.Headless tests share a single xUnit collection
  instead of contending with the EF-Core test host.
- Adds BlogAclApiTests (a brand-new behavioural layer over
  POST /api/v1/blogacl) and the fakes it relies on
  (BlogApiTestFakes, BlogPostAuthorDtoTests, AddCircleMember
  DialogTests); pulls UserId-through-OIDC-sub path into
  BearerScopeTests / FakeAuthorizingBrowser /
  OidcStubAuthority.
- App.axaml.cs gets a small PushPageAsync touch-up the new
  tests rely on.
- Drops UnitTest1.cs (xUnit scaffold, never used).

Yavsc.Blogs.Tests — SQLite instead of InMemory
- Bumps Yavsc.Blogs.Tests.csproj on
  Microsoft.EntityFrameworkCore.Sqlite and rewrites
  BlogsWebServerFixture to hold a single shared
  SqliteConnection (Cache=Shared) for the fixture lifetime,
  with a sync Dispose close to dodge async teardown hangs.
  Reason: the EF Core InMemory provider silently ignores FKs,
  which masked the kind of bug we are about to pin in the
  ACL tests. SQLite enforces them, so any future INSERT that
  forgets to seed its parent rows fails loudly here instead
  of passing the test and breaking prod.
- PublishEndpointTests and BlogApiSmokeTests get a one-line
  tweak to follow the new connection lifecycle.

Foreign-key fallout: seed the default user in the fixture
- Adds BlogsWebServerFixture.SeedUser(userName). Now that
  SQLite enforces BlogPost.AuthorId → AspNetUsers.Id, every
  test that POST/PUT/DELETE a BlogPost and sends AuthorId=
  'tester' in the payload needs an AspNetUsers row to satisfy
  the FK or it returns 500 with SQLite Error 19.
- BlogApiTests wraps the existing ResetDatabase with a
  ResetAndSeedDefaultUser helper for the six mutating tests;
  the four GET-only and ModelState-only tests keep the bare
  ResetDatabase.
- Side benefit: every test in Yavsc.Blogs.Tests now finishes
  cleanly instead of hanging at teardown — previously a stuck
  test held the shared SqliteConnection open and the next
  tests waited indefinitely.

Verified: dotnet test src/Yavsc.Blogs.Tests passes 25/25
green from a clean run, no fixture teardown hang.

src/Yavsc.Blogs.Tests/BlogApiSmokeTests.cs

@@ -12,6 +12,7 @@ namespace Yavsc.Blogs.Tests;
 /// surface. The first behavioural test (GET /api/v1/blog returns
 /// 200) lands in a follow-up commit.
 /// </summary>
+[Collection("Yavsc Blogs")]
 public sealed class BlogApiSmokeTests : IClassFixture<BlogsWebServerFixture>
 {
     private readonly BlogsWebServerFixture _fixture;