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/desktop/src-tauri/Cargo.lock

@@ -2687,6 +2687,7 @@ dependencies = [
  "tauri-plugin-updater",
  "tokio",
  "url",
+ "window-vibrancy 0.7.1",
 ]
 
 [[package]]
@@ -4269,7 +4270,7 @@ dependencies = [
  "url",
  "webkit2gtk",
  "webview2-com",
- "window-vibrancy",
+ "window-vibrancy 0.6.0",
  "windows",
 ]
 
@@ -5461,6 +5462,21 @@ dependencies = [
  "windows-version",
 ]
 
+[[package]]
+name = "window-vibrancy"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "010797bd7c40396fbc59d3105089fed0885fe267a0ef4a0a4646df54e28647f6"
+dependencies = [
+ "objc2 0.6.3",
+ "objc2-app-kit",
+ "objc2-core-foundation",
+ "objc2-foundation 0.3.2",
+ "raw-window-handle",
+ "windows-sys 0.60.2",
+ "windows-version",
+]
+
 [[package]]
 name = "windows"
 version = "0.61.3"