refactor: simplify journal message types and implement GM rolls

Refactor journal message types to use flatter, more intuitive names:
- `narrative` becomes `chat`
- `article.reveal` becomes `reveal`
- `roll.request`/`roll.result` becomes a single `roll` type

Implement client-side dice roll resolution for GMs. When a GM uses
the `/roll` command, the result is now calculated locally using
`resolveRollPayload` before being sent to the stream, rather than
requesting a roll from the server.

src/components/stores/journalStream.ts

@@ -3,7 +3,7 @@
  *
  * Reactive state for a single session's message stream. Manages MQTT
  * connection, local message log, per-sender sequence tracking, and the
- * revealed-paths set (populated by the article.reveal reducer).
+ * revealed-paths set (populated by the link reducer).
  *
  * Session lifecycle (create/list/delete) is handled via MQTT retained
  * topics — ttrpg/$SESSIONS for the manifest and ttrpg/{id}/meta per session.
@@ -29,7 +29,7 @@ export interface JournalStreamState {
   /** Last sequence number per sender */
   senderSeq: Record<string, number>;
   /**
-   * Paths revealed by article.reveal messages.
+   * Paths revealed by link messages.
    * Populated during hydration and live receipt via the type's reducer.
    */
   revealedPaths: Set<string>;