v3: persist sidebar state via cookie, default closed on tablet/mobile

Sidebar open/close state is now saved to a cookie (neko_sidebar) so it
persists across page reloads. On first visit without a cookie, the
sidebar defaults to closed on tablet and mobile (<=1024px viewport)
and open on desktop. Removes the auto-open-on-resize behavior that
was overriding user preference.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

3 files changed, 61 insertions, 7 deletions

diff --git a/frontend-vanilla/src/main.ts b/frontend-vanilla/src/main.ts
index 7d75e14..3dc0574 100644
--- a/frontend-vanilla/src/main.ts
+++ b/frontend-vanilla/src/main.ts
@@ -87,11 +87,7 @@ export function attachLayoutListeners() {
     store.setSidebarVisible(false);
   });
 
-  window.addEventListener('resize', () => {
-    if (window.innerWidth > 768 && !store.sidebarVisible) {
-      store.setSidebarVisible(true);
-    }
-  });
+  // Sidebar state is persisted via cookie; no auto-open on resize
 
   // Collapsible sections
   document.querySelectorAll('.sidebar-section.collapsible h3').forEach(header => {
diff --git a/frontend-vanilla/src/store.test.ts b/frontend-vanilla/src/store.test.ts
index 33deb7f..799db83 100644
--- a/frontend-vanilla/src/store.test.ts
+++ b/frontend-vanilla/src/store.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, vi } from 'vitest';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
 import { Store } from './store';
 
 describe('Store', () => {
@@ -105,4 +105,44 @@ describe('Store', () => {
         expect(localStorage.getItem('neko-font-theme')).toBe('serif');
         expect(callback).toHaveBeenCalled();
     });
+
+    describe('sidebar cookie persistence', () => {
+        beforeEach(() => {
+            // Clear sidebar cookie
+            document.cookie = 'neko_sidebar=; path=/; max-age=0';
+        });
+
+        it('should persist sidebar state to cookie', () => {
+            const store = new Store();
+            store.setSidebarVisible(false);
+            expect(document.cookie).toContain('neko_sidebar=0');
+
+            store.setSidebarVisible(true);
+            expect(document.cookie).toContain('neko_sidebar=1');
+        });
+
+        it('should read sidebar state from cookie on init', () => {
+            document.cookie = 'neko_sidebar=1; path=/';
+            const store = new Store();
+            expect(store.sidebarVisible).toBe(true);
+
+            document.cookie = 'neko_sidebar=0; path=/';
+            const store2 = new Store();
+            expect(store2.sidebarVisible).toBe(false);
+        });
+
+        it('should default to closed on tablet/mobile when no cookie', () => {
+            // jsdom defaults innerWidth to 0, which is <= 1024
+            const store = new Store();
+            expect(store.sidebarVisible).toBe(false);
+        });
+
+        it('should emit sidebar-toggle when toggling', () => {
+            const store = new Store();
+            const callback = vi.fn();
+            store.on('sidebar-toggle', callback);
+            store.toggleSidebar();
+            expect(callback).toHaveBeenCalled();
+        });
+    });
 });
diff --git a/frontend-vanilla/src/store.ts b/frontend-vanilla/src/store.ts
index a23934a..c92059a 100644
--- a/frontend-vanilla/src/store.ts
+++ b/frontend-vanilla/src/store.ts
@@ -4,6 +4,23 @@ export type StoreEvent = 'feeds-updated' | 'tags-updated' | 'items-updated' | 'a
 
 export type FilterType = 'unread' | 'all' | 'starred';
 
+function getSidebarCookie(): boolean | null {
+    const match = document.cookie.split('; ').find(row => row.startsWith('neko_sidebar='));
+    if (!match) return null;
+    return match.split('=')[1] === '1';
+}
+
+function setSidebarCookie(visible: boolean) {
+    document.cookie = `neko_sidebar=${visible ? '1' : '0'}; path=/; max-age=31536000; SameSite=Lax`;
+}
+
+function getInitialSidebarVisible(): boolean {
+    const saved = getSidebarCookie();
+    if (saved !== null) return saved;
+    // Default: closed on tablet+mobile (<=1024px), open on desktop
+    return window.innerWidth > 1024;
+}
+
 export class Store extends EventTarget {
     feeds: Feed[] = [];
     tags: Category[] = [];
@@ -16,7 +33,7 @@ export class Store extends EventTarget {
     hasMore: boolean = true;
     theme: string = localStorage.getItem('neko-theme') || 'light';
     fontTheme: string = localStorage.getItem('neko-font-theme') || 'default';
-    sidebarVisible: boolean = window.innerWidth > 768;
+    sidebarVisible: boolean = getInitialSidebarVisible();
 
     setFeeds(feeds: Feed[]) {
         this.feeds = feeds;
@@ -86,6 +103,7 @@ export class Store extends EventTarget {
 
     setSidebarVisible(visible: boolean) {
         this.sidebarVisible = visible;
+        setSidebarCookie(visible);
         this.emit('sidebar-toggle');
     }