feat: add spark table roll buttons for GMs

Introduces the ability for GMs to inject spark roll buttons into
tables that match existing spark completions.

- Replaces `linkPrefill` with a more flexible `actionPrefill` system
  to support multiple command types (e.g., `/link`, `/spark`).
- Implements `injectSparkButtons` to identify spark tables in the
  DOM and insert action buttons.
- Updates `JournalInput` to handle structured prefill actions.

src/components/journal/JournalInput.tsx

@@ -21,7 +21,7 @@ import {
   Match,
 } from "solid-js";
 import { sendMessage, useJournalStream } from "../stores/journalStream";
-import { linkPrefill, setLinkPrefill } from "../stores/reveal";
+import { actionPrefill, setActionPrefill } from "../stores/reveal";
 import { useJournalCompletions, ensureCompletions } from "./completions";
 import { resolveRollPayload } from "./types/roll";
 import { resolveSparkPayload } from "./types/spark";
@@ -97,12 +97,12 @@ export const JournalInput: Component = () => {
     void ensureCompletions();
   });
 
-  // Listen for /link prefill requests from article headings
+  // Listen for action prefill requests from article buttons (heading /link, spark table roll, etc.)
   createEffect(() => {
-    const prefilled = linkPrefill();
-    if (prefilled) {
-      setText(prefilled);
-      setLinkPrefill(null);
+    const action = actionPrefill();
+    if (action) {
+      setText(`${action.command} ${action.text}`);
+      setActionPrefill(null);
       textareaRef?.focus();
     }
   });
@@ -498,7 +498,7 @@ export const JournalInput: Component = () => {
             onKeyDown={handleKeyDown}
             placeholder={
               isGm()
-                ? "Type a message, or /roll or /link..."
+                ? "Type a message, or /roll, /spark, or /link..."
                 : "Type a message..."
             }
             rows={2}