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/views/git/BranchIntegrationSection.tsx

@@ -198,13 +198,13 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
       {operationCompleted ? (
         mode === 'dialog' ? (
           <DialogFooter>
-            <Button variant="default" size="sm" className="h-7 px-2 py-0" onClick={handleClose}>
+            <Button variant="default" size="sm" onClick={handleClose}>
               {hasError ? 'Close' : 'Done'}
             </Button>
           </DialogFooter>
         ) : (
           <div className="flex justify-end">
-            <Button variant="default" size="sm" className="h-7 px-2 py-0" onClick={handleClose}>
+            <Button variant="default" size="sm" onClick={handleClose}>
               {hasError ? 'Close' : 'Done'}
             </Button>
           </div>
@@ -284,7 +284,7 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
         </p>
         <DropdownMenu open={branchDropdownOpen} onOpenChange={setBranchDropdownOpen} modal={false}>
           <DropdownMenuTrigger asChild>
-            <Button variant="outline" className="w-full justify-between h-10">
+            <Button variant="outline" size="lg" className="w-full justify-between">
               <span className={cn('truncate', !selectedBranch && 'text-muted-foreground')}>
                 {selectedBranch || 'Select a branch...'}
               </span>
@@ -353,7 +353,7 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
 
       {mode === 'dialog' ? (
         <DialogFooter className="gap-2 pt-1">
-          <Button variant="ghost" size="sm" className="h-7 px-2 py-0" onClick={handleCancel}>
+          <Button variant="ghost" size="sm" onClick={handleCancel}>
             Cancel
           </Button>
           <Button
@@ -361,7 +361,7 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
             size="sm"
             onClick={handleConfirm}
             disabled={!selectedBranch}
-            className="h-7 px-2 py-0 gap-1.5"
+            className="gap-1.5"
           >
             {operation === 'merge' ? (
               <>
@@ -378,11 +378,11 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
         </DialogFooter>
       ) : (
         <div className="flex items-center gap-2 pt-1">
-          <Button variant="ghost" size="sm" className="h-7 px-2 py-0" onClick={handleCancel} disabled={isDisabled}>
+          <Button variant="destructive" size="sm" onClick={handleCancel} disabled={isDisabled}>
             Reset
           </Button>
           <div className="flex-1" />
-          <Button variant="default" size="sm" className="h-7 px-2 py-0" onClick={handleConfirm} disabled={isDisabled || !selectedBranch}>
+          <Button variant="default" size="sm" onClick={handleConfirm} disabled={isDisabled || !selectedBranch}>
             {operation === 'merge' ? 'Merge' : 'Rebase'}
           </Button>
         </div>
@@ -416,7 +416,7 @@ export const BranchIntegrationSection: React.FC<BranchIntegrationSectionProps> =
           <Button
             variant="outline"
             size="sm"
-            className="h-7 px-2 py-0 gap-1.5"
+            className="gap-1.5"
             onClick={handleOpenDialog}
             disabled={isDisabled}
           >