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/src/BigRegister.Api/Data/BriefStore.cs

@@ -95,6 +95,19 @@ public static class BriefStore
         lock (_gate) _byOwner.Clear();
     }
 
+    /// Demo affordance: drop the owner's brief and create a fresh one. No guards — a
+    /// "start over" for the showcase, not a real workflow transition.
+    public static BriefEntity ResetAndCreate(string owner)
+    {
+        lock (_gate)
+        {
+            _byOwner.Remove(owner);
+            var created = BriefSeed.NewBrief(owner);
+            _byOwner[owner] = created;
+            return created;
+        }
+    }
+
     // Approve/reject share the guard: must be submitted, and the approver must differ
     // from the drafter (a drafter cannot approve their own letter).
     private static (Outcome, BriefEntity?) Review(string owner, string actingId, Func<BriefEntity, BriefStatusDto> next)
@@ -142,11 +155,20 @@ public static class BriefSeed
         TemplateId = TemplateId,
         DrafterId = BriefStore.DrafterId,
         Placeholders = Placeholders,
+        // aanhef + slot are predefined, locked template text (prefilled); the drafter
+        // composes only the unlocked `kern`. Save trusts the FE not to mutate locked
+        // sections (FE-authoritative for this POC — the FE reducer also refuses it).
         Sections = new()
         {
-            new("aanhef", "Aanhef", true, new List<LetterBlockDto>()),
+            new("aanhef", "Aanhef", true, new List<LetterBlockDto>
+            {
+                new("freeText", "aanhef-1", Block(T("Geachte heer/mevrouw "), P("naam_zorgverlener"), T(","))),
+            }, Locked: true),
             new("kern", "Kern van het besluit", true, new List<LetterBlockDto>()),
-            new("slot", "Slot", false, new List<LetterBlockDto>()),
+            new("slot", "Slot", false, new List<LetterBlockDto>
+            {
+                new("freeText", "slot-1", Block(T("Met vriendelijke groet,"))),
+            }, Locked: true),
         },
         Status = new BriefStatusDto("draft"),
     };