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/mcp/McpDropdown.tsx

@@ -94,12 +94,12 @@ export const McpDropdownContent: React.FC<McpDropdownContentProps> = ({ active,
 
   return (
     <div className={cn('w-full', className)}>
-      <div className="sticky top-0 z-20 bg-[var(--surface-elevated)] border-b border-[var(--interactive-border)]">
-        <div className="flex items-center justify-between gap-3 px-2 py-2.5">
-          <div className="min-w-0 flex items-center gap-2">
+      <div className="border-b border-[var(--interactive-border)]">
+        <div className="flex items-center justify-between gap-3 px-4 py-2.5">
+          <div className="min-w-0 flex items-baseline gap-2">
             <div className="typography-ui-header font-semibold text-foreground">MCP Servers</div>
             {directory && (
-              <div className="truncate typography-ui-label text-muted-foreground">
+              <div className="truncate typography-micro text-muted-foreground">
                 {directory.split('/').pop() || directory}
               </div>
             )}
@@ -116,7 +116,7 @@ export const McpDropdownContent: React.FC<McpDropdownContentProps> = ({ active,
         </div>
       </div>
 
-      <div className="max-h-64 overflow-y-auto py-1">
+      <div className="max-h-64 overflow-y-auto py-2">
         {sortedNames.map((serverName) => {
           const serverStatus = status[serverName];
           const tone = statusTone(serverStatus);
@@ -127,7 +127,7 @@ export const McpDropdownContent: React.FC<McpDropdownContentProps> = ({ active,
           return (
             <div
               key={serverName}
-              className="flex items-center justify-between gap-2 px-2 py-1.5 rounded-lg hover:bg-interactive-hover/50"
+              className="flex items-center justify-between gap-2 px-4 py-1.5 rounded-lg hover:bg-interactive-hover/50"
             >
               <div className="min-w-0 flex-1">
                 <div className="flex items-center gap-2 min-w-0">
@@ -174,7 +174,7 @@ export const McpDropdownContent: React.FC<McpDropdownContentProps> = ({ active,
         })}
 
         {sortedNames.length === 0 && (
-          <div className="px-2 py-3 typography-ui-label text-muted-foreground text-center">
+          <div className="px-4 py-5 typography-ui-label text-muted-foreground text-center">
             Configure MCP servers in Opencode config.
           </div>
         )}