Add character aliases/identity links, move-beats, keyboard help overlay

- Characters can carry aliases and be linked as the same underlying
  person (canonical SameCharacterAsId, optional reveal chapter/note),
  surfaced through the API, MCP tools, agent toolset, and web UI.
- Characters page redesigned as a filterable/sortable table (name+
  aliases, role, importance, occupation, tags) instead of a sidebar
  list, to stay usable as the cast grows.
- Beats can be moved between chapters (BeatService.MoveAsync + MCP/
  agent tool + endpoint).
- Add a keyboard-shortcuts help overlay (HelpButton/HelpOverlayContext)
  wired into the project layout.
- CLAUDE.md: require every frontend component to carry a unique id
  attribute; apply it to CharacterMultiSelect and MarkdownEditor.

src/Novelly.Api/Beats/BeatService.cs

@@ -17,7 +17,8 @@ public class BeatService(
     IModelValidator<CreateBeatRequest> createValidator,
     IModelValidator<UpdateBeatRequest> updateValidator,
     IModelValidator<ReorderBeatsRequest> reorderValidator,
-    IModelValidator<AssignCharacterToBeatsRequest> assignCharacterValidator)
+    IModelValidator<AssignCharacterToBeatsRequest> assignCharacterValidator,
+    IModelValidator<MoveBeatsRequest> moveValidator)
 {
     public async Task<IReadOnlyList<Beat>> ListAsync(Guid chapterId, CancellationToken ct = default)
     {
@@ -266,6 +267,62 @@ public class BeatService(
         return await ListAsync(chapterId, ct);
     }
 
+    public async Task<IReadOnlyList<Beat>?> MoveAsync(
+        Guid chapterId, MoveBeatsRequest request, CancellationToken ct = default)
+    {
+        Guard.Default(chapterId, nameof(chapterId));
+        Guard.Null(request, nameof(request));
+        moveValidator.Validate(request).ThrowIfInvalid(logger);
+
+        logger.LogInformation(
+            "Moving {Count} beats from chapter {ChapterId} to chapter {TargetChapterId}",
+            request.BeatIds.Count, chapterId, request.TargetChapterId);
+
+        var chapter = await db.Chapters.FirstOrDefaultAsync(c => c.Id == chapterId, ct);
+        if (chapter is null)
+        {
+            logger.LogWarning("Rejected beat move: chapter {ChapterId} not found", chapterId);
+            return null;
+        }
+
+        var targetChapter = await db.Chapters.FirstOrDefaultAsync(
+            c => c.Id == request.TargetChapterId && c.ProjectId == chapter.ProjectId, ct);
+        if (targetChapter is null)
+        {
+            logger.LogWarning(
+                "Rejected beat move: target chapter {TargetChapterId} not found in project {ProjectId}",
+                request.TargetChapterId, chapter.ProjectId);
+            return null;
+        }
+
+        await access.RequireAsync(chapter.ProjectId, ProjectPermission.Write, ct);
+
+        var beats = await Query().Where(b => b.ChapterId == chapterId && request.BeatIds.Contains(b.Id)).ToListAsync(ct);
+        var missing = request.BeatIds.Where(id => beats.All(b => b.Id != id)).ToList();
+        if (missing.Count > 0)
+        {
+            logger.LogWarning("Rejected beat move: chapter {ChapterId} referenced missing beat {BeatId}", chapterId, missing[0]);
+            return null;
+        }
+
+        if (chapterId == request.TargetChapterId)
+        {
+            return beats;
+        }
+
+        var nextSortOrder = await NextSortOrderAsync(request.TargetChapterId, ct);
+        foreach (var beatId in request.BeatIds)
+        {
+            var beat = beats.Single(b => b.Id == beatId);
+            beat.ChapterId = request.TargetChapterId;
+            beat.SortOrder = nextSortOrder++;
+            beat.UpdatedAt = DateTimeOffset.UtcNow;
+        }
+
+        await db.SaveChangesAsync(ct);
+        return beats;
+    }
+
     private async Task<List<Character>> ResolveCharactersAsync(Guid projectId, IReadOnlyList<Guid> characterIds, CancellationToken ct)
     {
         logger.LogDebug("Resolving {Count} characters for project {ProjectId}", characterIds.Count, projectId);