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/chat/hooks/useChatTimelineController.ts

@@ -51,6 +51,7 @@ export interface UseChatTimelineControllerResult {
     loadEarlier: () => Promise<void>;
     revealBufferedTurns: () => Promise<boolean>;
     resumeToBottom: () => void;
+    resumeToBottomInstant: () => void;
     scrollToTurn: (turnId: string, options?: { behavior?: ScrollBehavior }) => Promise<boolean>;
     scrollToMessage: (messageId: string, options?: { behavior?: ScrollBehavior }) => Promise<boolean>;
     captureViewportAnchor: () => ViewportAnchor | null;
@@ -394,6 +395,14 @@ export const useChatTimelineController = ({
         scrollToBottom({ force: true });
     }, [scrollToBottom]);
 
+    const resumeToBottomInstant = React.useCallback(() => {
+        const nextStart = getInitialTurnStart(turnModelRef.current.turnCount);
+        setTurnStart(nextStart);
+        setPendingRevealWork(false);
+        setIsLoadingOlder(false);
+        scrollToBottom({ instant: true, force: true });
+    }, [scrollToBottom]);
+
     const handleActiveTurnChange = React.useCallback((turnId: string | null) => {
         setActiveTurnId(turnId);
     }, []);
@@ -411,6 +420,7 @@ export const useChatTimelineController = ({
         loadEarlier,
         revealBufferedTurns,
         resumeToBottom,
+        resumeToBottomInstant,
         scrollToTurn,
         scrollToMessage,
         captureViewportAnchor,