Major UI refresh: sidebar redesign, theme expansion, and chat performance optimizations (#706)

## Summary
Complete sidebar redesign and comprehensive UI polish pass with performance optimizations, theme system refinements, and desktop integration improvements.

## Key Changes

**Sidebar & Navigation Redesign**
- Redesigned sessions sidebar layout with unified button primitives
- Added activity sections with project grouping and improved session organization
- Refined sidebar corners, spacing, and visual hierarchy
- Removed NavRail component in favor of streamlined sidebar
- Stabilized sessions bar toggle position in fullscreen mode

**Performance Optimizations**
- Reduced chat streaming CPU usage and storage churn
- Optimized task tool polling and live timers with debouncing
- Prevented chat state races and reduced background request load
- Debounced draft writes and coalesced session reloads
- Optimized message store updates and turn tracking

**Theme & Visual System**
- Added theme-aware window corners (desktop) and border radius tokens
- Introduced glassmorphism effects on desktop sidebar
- Added backdrop blur to UI elements

**Chat Experience**
- Added session-based permission auto-accept toggle in chat input
- Polished permission shield UX with improved icon sizing and spacing
- Fixed chat scroll-to-bottom behavior and timeline tracking
- Enhanced tool output display with better path label detection
- Removed duplicate draft context details in chat header
- Added text selection menu to chat messages

**Git Improvements**
- Refreshed git history visual design with cleaner dividers
- Added remote removal action in sync selector
- Stabilized git polling to prevent excessive requests
- Improved tool output rendering for git operations

**Settings & Panels**
- Fixed mobile scrolling on settings pages
- Made outside-click settings close instantly
- Reduced settings load churn and CPU spikes
- Improved services dropdown layout and spacing
- Softened panel resize handles

**Desktop Integration**
- Synced macOS window theme with app theme
- Restored window dragging in sidebar header zones
- Fixed system window corners on macOS
- Improved header session metadata and action controls

**Button & Component Standardization**
- Unified button primitives across all components
- Standardized destructive action patterns
- Removed unused button variants (button-large, button-small)
- Aligned context tab close hit areas

---------

Co-authored-by: Iuliia Ivashko <yulia.ivashko@gmail.com>

packages/ui/src/components/layout/Sidebar.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { cn } from '@/lib/utils';
 import { ErrorBoundary } from '../ui/ErrorBoundary';
 import { useUIStore } from '@/stores/useUIStore';
+import { isDesktopShell } from '@/lib/desktop';
 
 export const SIDEBAR_CONTENT_WIDTH = 250;
 const SIDEBAR_MIN_WIDTH = 250;
@@ -11,10 +12,12 @@ interface SidebarProps {
     isOpen: boolean;
     isMobile: boolean;
     children: React.ReactNode;
+    className?: string;
 }
 
-export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children }) => {
+export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children, className }) => {
     const { sidebarWidth, setSidebarWidth } = useUIStore();
+    const isDesktopApp = React.useMemo(() => isDesktopShell(), []);
     const [isResizing, setIsResizing] = React.useState(false);
     const startXRef = React.useRef(0);
     const startWidthRef = React.useRef(sidebarWidth || SIDEBAR_CONTENT_WIDTH);
@@ -115,9 +118,12 @@ export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children })
             ref={sidebarRef}
             className={cn(
                 'relative flex h-full overflow-hidden border-r border-border/40',
-                'bg-sidebar/50',
+                isDesktopApp
+                    ? 'bg-[color:var(--sidebar-overlay-strong)] backdrop-blur supports-[backdrop-filter]:bg-[color:var(--sidebar-overlay-soft)]'
+                    : 'bg-sidebar',
                 isResizing ? 'transition-none' : 'transition-[width] duration-300 ease-in-out',
-                !isOpen && 'border-r-0'
+                !isOpen && 'border-r-0',
+                className,
             )}
             style={{
                 width: 'var(--oc-left-sidebar-width)',
@@ -131,8 +137,8 @@ export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children })
             {isOpen && (
                 <div
                     className={cn(
-                        'absolute right-0 top-0 z-20 h-full w-[4px] cursor-col-resize hover:bg-primary/50 transition-colors',
-                        isResizing && 'bg-primary'
+                        'absolute right-0 top-0 z-20 h-full w-[3px] cursor-col-resize hover:bg-[var(--interactive-border)]/80 transition-colors',
+                        isResizing && 'bg-[var(--interactive-border)]'
                     )}
                     onPointerDown={handlePointerDown}
                     onPointerMove={handlePointerMove}