feat(brief): locked sections, list formatting, auto/manual placeholder chips

- brief.machine: reducer refuses edits to locked (predefined) sections as
  defense-in-depth; LetterSection gains a `locked` flag
- rich-text: paragraphs gain optional `list` kind; editor gets bullet/numbered
  list buttons, keyboard shortcuts, and backspace-deletes-adjacent-chip
- placeholder chips distinguish auto-resolvable (grey) vs manual (yellow), in
  both the editor and the read-only preview
- fix: preview chip now renders matching {…} braces (was a one-sided ⌗ glyph),
  aligned with the editor's chip styling

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/BriefEndpointTests.cs

@@ -49,7 +49,14 @@ public class BriefEndpointTests(WebApplicationFactory<Program> factory) : IClass
         var brief = await Get();
         Assert.Equal("draft", brief.Status.Tag);
         Assert.Equal(new[] { "aanhef", "kern", "slot" }, brief.Sections.Select(s => s.SectionKey));
-        Assert.All(brief.Sections, s => Assert.Empty(s.Blocks));
+        // aanhef + slot are locked, predefined and prefilled; only kern is editable + empty.
+        var aanhef = brief.Sections.Single(s => s.SectionKey == "aanhef");
+        Assert.True(aanhef.Locked);
+        Assert.NotEmpty(aanhef.Blocks);
+        Assert.True(brief.Sections.Single(s => s.SectionKey == "slot").Locked);
+        var kern = brief.Sections.Single(s => s.SectionKey == "kern");
+        Assert.False(kern.Locked);
+        Assert.Empty(kern.Blocks);
 
         var view = await _client.GetFromJsonAsync<BriefViewDto>("/api/v1/brief");
         // global passages + the arts-scoped one; no other-beroep passages leak in.
@@ -134,4 +141,22 @@ public class BriefEndpointTests(WebApplicationFactory<Program> factory) : IClass
         res.EnsureSuccessStatusCode();
         Assert.Equal("sent", (await res.Content.ReadFromJsonAsync<BriefDto>())!.Status.Tag);
     }
+
+    [Fact]
+    public async Task Reset_recreates_a_fresh_draft_with_locked_prefilled_sections()
+    {
+        var brief = await Get();
+        // Advance out of draft so the reset back to draft is observable.
+        await _client.PutAsJsonAsync("/api/v1/brief", FilledFrom(brief));
+        await _client.SendAsync(Post("/api/v1/brief/submit"));
+
+        var res = await _client.SendAsync(Post("/api/v1/brief/reset"));
+        res.EnsureSuccessStatusCode();
+        var view = await res.Content.ReadFromJsonAsync<BriefViewDto>();
+        Assert.Equal("draft", view!.Brief.Status.Tag);
+        var aanhef = view.Brief.Sections.Single(s => s.SectionKey == "aanhef");
+        Assert.True(aanhef.Locked);
+        Assert.NotEmpty(aanhef.Blocks);
+        Assert.Empty(view.Brief.Sections.Single(s => s.SectionKey == "kern").Blocks);
+    }
 }