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/desktop/DesktopHostSwitcher.tsx

@@ -681,14 +681,14 @@ export function DesktopHostSwitcherDialog({
   const content = (
     <>
       {embedded ? (
-        <div className="flex-shrink-0 border-b border-[var(--interactive-border)] bg-[var(--surface-elevated)] px-2 py-2.5">
+        <div className="flex-shrink-0 border-b border-[var(--interactive-border)] px-3 py-2">
           <div className="flex items-center justify-between gap-3">
-            <div className="min-w-0 flex items-center gap-2">
-              <span className="typography-ui-header font-semibold text-foreground">Current</span>
-              <span className="max-w-[9rem] truncate typography-ui-label text-muted-foreground">{redactSensitiveUrl(current.label)}</span>
-              <span className="text-muted-foreground">•</span>
-              <span className="typography-ui-header font-semibold text-foreground">Default</span>
-              <span className="max-w-[9rem] truncate typography-ui-label text-muted-foreground">{redactSensitiveUrl(currentDefaultLabel)}</span>
+            <div className="min-w-0 flex items-baseline gap-1.5 typography-ui-label">
+              <span className="font-medium text-foreground">Current</span>
+              <span className="max-w-[9rem] truncate text-muted-foreground">{redactSensitiveUrl(current.label)}</span>
+              <span className="text-muted-foreground/50">•</span>
+              <span className="font-medium text-foreground">Default</span>
+              <span className="max-w-[9rem] truncate text-muted-foreground">{redactSensitiveUrl(currentDefaultLabel)}</span>
             </div>
             <button
               type="button"
@@ -741,14 +741,12 @@ export function DesktopHostSwitcherDialog({
       )}
 
         {tauriAvailable && (
-          <div className="flex-shrink-0 rounded-md border border-[var(--interactive-border)] bg-[var(--surface-elevated)] px-2.5 py-2">
-            <div className="flex items-center justify-between gap-2">
-              <span className="typography-micro text-muted-foreground">Need SSH instances? Manage them in Settings.</span>
-              <Button type="button" variant="outline" size="sm" onClick={openRemoteInstancesSettings}>
-                <RiSettings3Line className="h-4 w-4" />
-                Remote SSH
-              </Button>
-            </div>
+          <div className="flex-shrink-0 flex items-center justify-between gap-2 px-2.5 py-1.5">
+            <span className="typography-micro text-muted-foreground">Need SSH instances?<br />Manage them in Settings.</span>
+            <Button type="button" variant="ghost" size="sm" onClick={openRemoteInstancesSettings}>
+              <RiSettings3Line className="h-4 w-4" />
+              Remote SSH
+            </Button>
           </div>
         )}