feat(ui): add collapsible sidebar functionality

Implement a collapsible sidebar feature for the desktop view that allows
users to toggle between expanded and collapsed states. The sidebar state
is persisted using localStorage to maintain user preference across
sessions.

Changes include:
- Add sidebarVisible state with localStorage persistence
- Import Sparkles and SettingsIcon from lucide-react
- Implement smooth transition animation (300ms) for sidebar collapse
- Add collapsed sidebar view with icon-only navigation buttons
- Pass onToggleSidebar prop to Sidebar component
- Adjust sidebar width dynamically (80 -> 14 when collapsed)

This improves the user experience by providing more screen real estate
for the main content area when needed, while keeping quick access to
essential navigation through the collapsed icon view.

src/App.jsx

@@ -20,6 +20,7 @@
 
 import React, { useState, useEffect, useCallback } from 'react';
 import { BrowserRouter as Router, Routes, Route, useNavigate, useParams } from 'react-router-dom';
+import { Settings as SettingsIcon, Sparkles } from 'lucide-react';
 import Sidebar from './components/Sidebar';
 import MainContent from './components/MainContent';
 import MobileNav from './components/MobileNav';
@@ -60,6 +61,7 @@ function AppContent() {
   const [showThinking, setShowThinking] = useLocalStorage('showThinking', true);
   const [autoScrollToBottom, setAutoScrollToBottom] = useLocalStorage('autoScrollToBottom', true);
   const [sendByCtrlEnter, setSendByCtrlEnter] = useLocalStorage('sendByCtrlEnter', false);
+  const [sidebarVisible, setSidebarVisible] = useLocalStorage('sidebarVisible', true);
   // Session Protection System: Track sessions with active conversations to prevent
   // automatic project updates from interrupting ongoing chats. When a user sends
   // a message, the session is marked as "active" and project updates are paused
@@ -734,28 +736,78 @@ function AppContent() {
     <div className="fixed inset-0 flex bg-background">
       {/* Fixed Desktop Sidebar */}
       {!isMobile && (
-        <div className="w-80 flex-shrink-0 border-r border-border bg-card">
+        <div
+          className={`flex-shrink-0 border-r border-border bg-card transition-all duration-300 ${
+            sidebarVisible ? 'w-80' : 'w-14'
+          }`}
+        >
           <div className="h-full overflow-hidden">
-            <Sidebar
-              projects={projects}
-              selectedProject={selectedProject}
-              selectedSession={selectedSession}
-              onProjectSelect={handleProjectSelect}
-              onSessionSelect={handleSessionSelect}
-              onNewSession={handleNewSession}
-              onSessionDelete={handleSessionDelete}
-              onProjectDelete={handleProjectDelete}
-              isLoading={isLoadingProjects}
-              onRefresh={handleSidebarRefresh}
-              onShowSettings={() => setShowSettings(true)}
-              updateAvailable={updateAvailable}
-              latestVersion={latestVersion}
-              currentVersion={currentVersion}
-              releaseInfo={releaseInfo}
-              onShowVersionModal={() => setShowVersionModal(true)}
-              isPWA={isPWA}
-              isMobile={isMobile}
-            />
+            {sidebarVisible ? (
+              <Sidebar
+                projects={projects}
+                selectedProject={selectedProject}
+                selectedSession={selectedSession}
+                onProjectSelect={handleProjectSelect}
+                onSessionSelect={handleSessionSelect}
+                onNewSession={handleNewSession}
+                onSessionDelete={handleSessionDelete}
+                onProjectDelete={handleProjectDelete}
+                isLoading={isLoadingProjects}
+                onRefresh={handleSidebarRefresh}
+                onShowSettings={() => setShowSettings(true)}
+                updateAvailable={updateAvailable}
+                latestVersion={latestVersion}
+                currentVersion={currentVersion}
+                releaseInfo={releaseInfo}
+                onShowVersionModal={() => setShowVersionModal(true)}
+                isPWA={isPWA}
+                isMobile={isMobile}
+                onToggleSidebar={() => setSidebarVisible(false)}
+              />
+            ) : (
+              /* Collapsed Sidebar */
+              <div className="h-full flex flex-col items-center py-4 gap-4">
+                {/* Expand Button */}
+                <button
+                  onClick={() => setSidebarVisible(true)}
+                  className="p-2 hover:bg-accent rounded-md transition-colors duration-200 group"
+                  aria-label="Show sidebar"
+                  title="Show sidebar"
+                >
+                  <svg
+                    className="w-5 h-5 text-foreground group-hover:scale-110 transition-transform"
+                    fill="none"
+                    stroke="currentColor"
+                    viewBox="0 0 24 24"
+                  >
+                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M13 5l7 7-7 7M5 5l7 7-7 7" />
+                  </svg>
+                </button>
+
+                {/* Settings Icon */}
+                <button
+                  onClick={() => setShowSettings(true)}
+                  className="p-2 hover:bg-accent rounded-md transition-colors duration-200"
+                  aria-label="Settings"
+                  title="Settings"
+                >
+                  <SettingsIcon className="w-5 h-5 text-muted-foreground hover:text-foreground transition-colors" />
+                </button>
+
+                {/* Update Indicator */}
+                {updateAvailable && (
+                  <button
+                    onClick={() => setShowVersionModal(true)}
+                    className="relative p-2 hover:bg-accent rounded-md transition-colors duration-200"
+                    aria-label="Update available"
+                    title="Update available"
+                  >
+                    <Sparkles className="w-5 h-5 text-blue-500" />
+                    <span className="absolute top-1 right-1 w-2 h-2 bg-blue-500 rounded-full animate-pulse" />
+                  </button>
+                )}
+              </div>
+            )}
           </div>
         </div>
       )}
@@ -805,6 +857,7 @@ function AppContent() {
               onShowVersionModal={() => setShowVersionModal(true)}
               isPWA={isPWA}
               isMobile={isMobile}
+              onToggleSidebar={() => setSidebarVisible(false)}
             />
           </div>
         </div>