feat(utils): add branch storage utility

src/lib/utils/branchStorage.ts

@@ -0,0 +1,77 @@
+import { browser } from "$app/environment";
+
+export interface BranchStateSnapshot {
+	messageId: string;
+	personaId: string;
+	personaName: string;
+}
+
+export interface BranchTrackingSnapshot {
+	messageId: string | null;
+	personaId: string | null;
+}
+
+const BRANCH_STATE_PREFIX = "branchState_";
+const BRANCH_TRACKING_PREFIX = "branchTracking_";
+
+function getBranchStateKey(conversationId: string): string {
+	return `${BRANCH_STATE_PREFIX}${conversationId}`;
+}
+
+function getBranchTrackingKey(conversationId: string): string {
+	return `${BRANCH_TRACKING_PREFIX}${conversationId}`;
+}
+
+export function loadBranchState(conversationId: string): BranchStateSnapshot | null {
+	if (!browser) return null;
+	const stored = localStorage.getItem(getBranchStateKey(conversationId));
+	if (!stored) return null;
+	try {
+		return JSON.parse(stored) as BranchStateSnapshot;
+	} catch {
+		localStorage.removeItem(getBranchStateKey(conversationId));
+		return null;
+	}
+}
+
+export function persistBranchState(
+	conversationId: string,
+	state: BranchStateSnapshot | null
+): void {
+	if (!browser) return;
+	const key = getBranchStateKey(conversationId);
+	if (state) {
+		localStorage.setItem(key, JSON.stringify(state));
+	} else {
+		localStorage.removeItem(key);
+	}
+}
+
+export function loadBranchTracking(conversationId: string): BranchTrackingSnapshot {
+	if (!browser) return { messageId: null, personaId: null };
+	const stored = localStorage.getItem(getBranchTrackingKey(conversationId));
+	if (!stored) return { messageId: null, personaId: null };
+	try {
+		const parsed = JSON.parse(stored) as BranchTrackingSnapshot;
+		return {
+			messageId: parsed.messageId ?? null,
+			personaId: parsed.personaId ?? null,
+		};
+	} catch {
+		localStorage.removeItem(getBranchTrackingKey(conversationId));
+		return { messageId: null, personaId: null };
+	}
+}
+
+export function persistBranchTracking(
+	conversationId: string,
+	tracking: BranchTrackingSnapshot | null
+): void {
+	if (!browser) return;
+	const key = getBranchTrackingKey(conversationId);
+	if (tracking && (tracking.messageId || tracking.personaId)) {
+		localStorage.setItem(key, JSON.stringify(tracking));
+	} else {
+		localStorage.removeItem(key);
+	}
+}