feat(ui): add conversation tree store for state management

src/lib/stores/conversationTree.ts

@@ -0,0 +1,40 @@
+import { writable } from "svelte/store";
+import type { Message } from "$lib/types/Message";
+
+interface ConversationTreeState {
+	conversationId: string | null;
+	messages: Message[];
+	activeMessageId: string | null; // Currently displayed message
+	branchedFrom: { messageId: string; personaId: string } | null;
+	activePath: string[]; // Linear path of message IDs visible in chat
+}
+
+const initialState: ConversationTreeState = {
+	conversationId: null,
+	messages: [],
+	activeMessageId: null,
+	branchedFrom: null,
+	activePath: [],
+};
+
+export const conversationTree = writable<ConversationTreeState>(initialState);
+
+export function setConversationTree(
+	conversationId: string,
+	messages: Message[],
+	activeMessageId?: string,
+	branchedFrom?: { messageId: string; personaId: string } | null,
+	activePath: string[] = []
+) {
+	conversationTree.set({
+		conversationId,
+		messages,
+		activeMessageId: activeMessageId ?? null,
+		branchedFrom: branchedFrom ?? null,
+		activePath,
+	});
+}
+
+export function clearConversationTree() {
+	conversationTree.set(initialState);
+}