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.

CLAUDE.md

@@ -58,6 +58,7 @@ Serilog console via `AddSerilog` (not `UseSerilog` — keeps OTel provider for A
 - DTOs are records; entities are classes.  DO NOT use Dto in names.
 - `PATCH` requests partial: null field = leave alone, empty string = clear. Keep new update endpoints consistent with `Patch.Apply`.
 - Enums cross wire as names, never ordinals
+- All frontend components should have an id attribute that identifies them uniquely.
 
 ## Testing