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/stores/useUIStore.ts

@@ -471,7 +471,6 @@ interface UIStore {
   isBottomTerminalExpanded: boolean;
   bottomTerminalHeight: number;
   hasManuallyResizedBottomTerminal: boolean;
-  isNavRailExpanded: boolean;
   isSessionSwitcherOpen: boolean;
   activeMainTab: MainTab;
   mainTabGuard: MainTabGuard | null;
@@ -589,8 +588,6 @@ interface UIStore {
   setBottomTerminalOpen: (open: boolean) => void;
   setBottomTerminalExpanded: (expanded: boolean) => void;
   setBottomTerminalHeight: (height: number) => void;
-  setNavRailExpanded: (expanded: boolean) => void;
-  toggleNavRail: () => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   setActiveMainTab: (tab: MainTab) => void;
   setMainTabGuard: (guard: MainTabGuard | null) => void;
@@ -703,7 +700,6 @@ export const useUIStore = create<UIStore>()(
         isBottomTerminalExpanded: false,
         bottomTerminalHeight: 300,
         hasManuallyResizedBottomTerminal: false,
-        isNavRailExpanded: false,
         isSessionSwitcherOpen: false,
         activeMainTab: 'chat',
         mainTabGuard: null,
@@ -739,7 +735,7 @@ export const useUIStore = create<UIStore>()(
         fontSize: 100,
         terminalFontSize: 13,
         padding: 100,
-        cornerRadius: 12,
+        cornerRadius: 18,
         inputBarOffset: 0,
         favoriteModels: [],
         hiddenModels: [],
@@ -1169,13 +1165,6 @@ export const useUIStore = create<UIStore>()(
           set({ bottomTerminalHeight: height, hasManuallyResizedBottomTerminal: true });
         },
 
-        setNavRailExpanded: (expanded) => {
-          set({ isNavRailExpanded: expanded });
-        },
-        toggleNavRail: () => {
-          set({ isNavRailExpanded: !get().isNavRailExpanded });
-        },
-
         setSessionSwitcherOpen: (open) => {
           set({ isSessionSwitcherOpen: open });
         },
@@ -1827,7 +1816,6 @@ export const useUIStore = create<UIStore>()(
           isBottomTerminalOpen: state.isBottomTerminalOpen,
           isBottomTerminalExpanded: state.isBottomTerminalExpanded,
           bottomTerminalHeight: state.bottomTerminalHeight,
-          isNavRailExpanded: state.isNavRailExpanded,
           isSessionSwitcherOpen: state.isSessionSwitcherOpen,
           activeMainTab: state.activeMainTab,
           sidebarSection: state.sidebarSection,