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/sections/usage/UsageSidebar.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import { ScrollableOverlay } from '@/components/ui/ScrollableOverlay';
 import { ProviderLogo } from '@/components/ui/ProviderLogo';
-import { ButtonSmall } from '@/components/ui/button-small';
+import { Button } from '@/components/ui/button';
 import { Checkbox } from '@/components/ui/checkbox';
 import { Tooltip, TooltipTrigger, TooltipContent } from '@/components/ui/tooltip';
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@/components/ui/select';
@@ -111,7 +111,7 @@ export const UsageSidebar: React.FC<UsageSidebarProps> = ({ onItemSelect }) => {
                 <SelectItem value="300000">5m</SelectItem>
               </SelectContent>
             </Select>
-            <ButtonSmall
+            <Button size="sm"
               variant="ghost"
               className="h-7 w-7 px-0 text-muted-foreground"
               onClick={() => fetchAllQuotas()}
@@ -120,7 +120,7 @@ export const UsageSidebar: React.FC<UsageSidebarProps> = ({ onItemSelect }) => {
               disabled={isLoading}
             >
               <RiRefreshLine className={cn('h-3.5 w-3.5', isLoading && 'animate-spin')} />
-            </ButtonSmall>
+            </Button>
           </div>
         </div>
         <div className="mt-2 flex items-center justify-between gap-2">