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>

docs/CUSTOM_THEMES.md

@@ -45,9 +45,9 @@ OpenChamber supports user-defined themes. Drop a JSON file into the themes direc
     "surface": {
       "background": "#100F0F",
       "foreground": "#CECDC3",
-      "muted": "#1C1B1A",
+      "muted": "#1C1B1A90",
       "mutedForeground": "#878580",
-      "elevated": "#1C1A19",
+      "elevated": "#1C1A1990",
       "elevatedForeground": "#CECDC3",
       "overlay": "#00000080",
       "subtle": "#1e1d1c"
@@ -194,10 +194,10 @@ OpenChamber supports user-defined themes. Drop a JSON file into the themes direc
     },
     "radius": {
       "none": "0",
-      "sm": "0.125rem",
-      "md": "0.375rem",
-      "lg": "0.5rem",
-      "xl": "0.75rem",
+      "sm": "0.325rem",
+      "md": "0.75rem",
+      "lg": "1.125rem",
+      "xl": "1.5rem",
       "full": "9999px"
     },
     "transitions": {
@@ -209,6 +209,10 @@ OpenChamber supports user-defined themes. Drop a JSON file into the themes direc
 }
 ```
 
+## Surface Alpha Requirement
+
+- `colors.surface.muted` and `colors.surface.elevated` must always use 90 alpha (`...90` in 8-digit hex, e.g. `#1C1B1A90`).
+
 ## Validation
 
 Themes are validated on load. Invalid themes are skipped with a console warning.