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/contexts/ThemeSystemContext.tsx

@@ -7,7 +7,7 @@ import React, {
 import { flushSync } from 'react-dom';
 import type { Theme, ThemeMode } from '@/types/theme';
 import type { DesktopSettings } from '@/lib/desktop';
-import { isDesktopLocalOriginActive, isVSCodeRuntime } from '@/lib/desktop';
+import { isDesktopLocalOriginActive, isTauriShell, isVSCodeRuntime } from '@/lib/desktop';
 import { CSSVariableGenerator } from '@/lib/theme/cssGenerator';
 import { updateDesktopSettings } from '@/lib/persistence';
 import {
@@ -218,6 +218,7 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
   });
   const isVSCode = useMemo(() => isVSCodeRuntime(), []);
   const isLocalDesktopOrigin = useMemo(() => isDesktopLocalOriginActive(), []);
+  const isDesktopShell = useMemo(() => isTauriShell(), []);
 
   const availableThemes = useMemo(() => {
     const merged: Theme[] = [];
@@ -573,6 +574,33 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
     });
   }, [currentTheme.metadata.id, currentTheme.metadata.variant, ensureThemeById, preferences.themeMode, preferences.lightThemeId, preferences.darkThemeId]);
 
+  useEffect(() => {
+    if (!isDesktopShell) {
+      return;
+    }
+
+    let cancelled = false;
+
+    void (async () => {
+      try {
+        const { invoke } = await import('@tauri-apps/api/core');
+        if (cancelled) {
+          return;
+        }
+        await invoke('desktop_set_window_theme', {
+          themeMode: preferences.themeMode,
+          themeVariant: currentTheme.metadata.variant,
+        });
+      } catch {
+        // noop
+      }
+    })();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [currentTheme.metadata.variant, isDesktopShell, preferences.themeMode]);
+
   useEffect(() => {
     if (typeof window === 'undefined') {
       return;