feat(fp): WP-27 — brief UX layer (undo/redo, standaardbrief, passage search, diff badges)

Brief letter-composition UX improvements:
- undo/redo history in the brief store (snapshot stacks, Ctrl/Cmd+Z) + retry-save
- "Standaardbrief invoegen" starter for empty sections; isDefault library passages
  (backend DTO/seed + adapter parse)
- passage-picker client-side search (rich-text textOf helper)
- rejection diff badges on the letter canvas + show/hide changes toggle
  (pure brief-diff domain fns + spec)

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

backend/src/BigRegister.Api/Contracts/Dtos.cs

@@ -139,7 +139,7 @@ public sealed record BriefStatusDto(
 
 public sealed record LibraryPassageDto(
     string PassageId, string Scope, string SectionKey, string Label,
-    RichTextBlockDto Content, int Version, string? Beroep = null);
+    RichTextBlockDto Content, int Version, string? Beroep = null, bool IsDefault = false);
 
 public sealed record BriefDto(
     string BriefId, string Beroep, string TemplateId,

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

@@ -226,10 +226,12 @@ public static class BriefSeed
         {
             new("p-aanhef-1", "global", "aanhef", "Standaard aanhef",
                 Block(T("Geachte heer/mevrouw "), P("naam_zorgverlener"), T(",")), 1),
+            // The "standaardbrief" kern starter set (IsDefault): one button drops these
+            // in when the kern is still empty (WP-27).
             new("p-kern-1", "global", "kern", "Beoordeling ontvangen",
-                Block(T("Op "), P("datum"), T(" hebben wij uw aanvraag beoordeeld.")), 1),
+                Block(T("Op "), P("datum"), T(" hebben wij uw aanvraag beoordeeld.")), 1, IsDefault: true),
             new("p-kern-arts", "beroep", "kern", "Toelichting arts",
-                Block(T("Als arts met BIG-nummer "), P("big_nummer"), T(" delen wij u het volgende mee.")), 1, Beroep: "arts"),
+                Block(T("Als arts met BIG-nummer "), P("big_nummer"), T(" delen wij u het volgende mee.")), 1, Beroep: "arts", IsDefault: true),
             new("p-slot-1", "global", "slot", "Standaard slot",
                 Block(T("Met vriendelijke groet,")), 1),
             // These reference a deprecated / not-fillable field so the linter's

backend/swagger.json

@@ -1720,6 +1720,9 @@
           "beroep": {
             "type": "string",
             "nullable": true
+          },
+          "isDefault": {
+            "type": "boolean"
           }
         },
         "additionalProperties": false