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/hooks/useScrollEngine.ts

@@ -38,6 +38,7 @@ const LERP_FACTOR = 0.14;
 
 // When the remaining distance is below this, snap exactly to bottom.
 const SNAP_EPSILON = 0.5;
+const FOLLOW_STABLE_FRAME_LIMIT = 8;
 
 export const useScrollEngine = ({
     containerRef,
@@ -81,6 +82,7 @@ export const useScrollEngine = ({
         if (followActiveRef.current) return; // already running
         followActiveRef.current = true;
         setIsFollowingBottom(true);
+        let stableFrames = 0;
 
         const tick = () => {
             const container = containerRef.current;
@@ -97,11 +99,18 @@ export const useScrollEngine = ({
 
             if (Math.abs(delta) <= SNAP_EPSILON) {
                 container.scrollTop = target;
-                // Don't stop — keep running so next content growth is caught immediately.
+                stableFrames += 1;
+                if (stableFrames >= FOLLOW_STABLE_FRAME_LIMIT) {
+                    followActiveRef.current = false;
+                    followRafRef.current = null;
+                    setIsFollowingBottom(false);
+                    return;
+                }
                 followRafRef.current = window.requestAnimationFrame(tick);
                 return;
             }
 
+            stableFrames = 0;
             container.scrollTop = current + delta * LERP_FACTOR;
             followRafRef.current = window.requestAnimationFrame(tick);
         };