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/ContextPanel.tsx

@@ -427,7 +427,7 @@ export const ContextPanel: React.FC = () => {
   const isFileTabActive = activeTab?.mode === 'file';
 
   const header = (
-    <header className="flex h-8 items-stretch border-b border-border/40">
+    <header className="flex h-10 items-stretch border-b border-transparent">
       <SortableTabsStrip
         items={tabItems}
         activeId={activeTab?.id ?? null}
@@ -450,8 +450,11 @@ export const ContextPanel: React.FC = () => {
           reorderContextPanelTabs(directoryKey, activeTabID, overTabID);
         }}
         layoutMode="scrollable"
+        variant="active-pill"
+        activePillLowercase={false}
+        activePillInsetClassName="gap-0.5 pt-0.5 pb-1.5"
       />
-      <div className="flex items-center gap-1 px-1.5">
+      <div className="flex items-end gap-1 px-1.5 pb-1.5">
         <Button
           type="button"
           variant="ghost"
@@ -515,8 +518,8 @@ export const ContextPanel: React.FC = () => {
       {!isExpanded && (
         <div
           className={cn(
-            'absolute left-0 top-0 z-20 h-full w-[4px] cursor-col-resize transition-colors hover:bg-primary/50',
-            isResizing && 'bg-primary'
+            'absolute left-0 top-0 z-20 h-full w-[3px] cursor-col-resize transition-colors hover:bg-[var(--interactive-border)]/80',
+            isResizing && 'bg-[var(--interactive-border)]'
           )}
           onPointerDown={handleResizeStart}
           onPointerMove={handleResizeMove}