feat: implement stat system and StatsView

Introduces a comprehensive stat management system including:
- `/stat` command support (set, del, roll)
- `StatsView` component for displaying grouped stat tables
- Formula evaluation for derived stats (supporting arithmetic and
  functions)
- Stat definition parsing from YAML blocks
- Permission checking for stat modification based on roles

src/components/stores/journalStream.ts

@@ -59,6 +59,12 @@ export interface JournalStreamState {
   brokerUrl: string | null;
   /** Active player list (keyed by player name) */
   players: Record<string, { role: string }>;
+  /**
+   * Stat values set via /stat set/del/roll commands.
+   * Key: stat key (e.g. "strength", "alice:hp"). Value: string.
+   * Populated during hydration and live receipt via the stat type's reducer.
+   */
+  stats: Record<string, string>;
 }
 
 export interface SessionMeta {
@@ -95,6 +101,7 @@ const [state, setState] = createStore<JournalStreamState>({
   myRole: (persisted.myRole as "gm" | "player" | "observer") || "gm",
   brokerUrl: persisted.brokerUrl,
   players: {},
+  stats: {},
 });
 
 // Sync initial URL params if they came from localStorage (not URL)