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/src/main.rs

@@ -24,6 +24,10 @@ use std::{
 };
 use tauri::utils::config::BackgroundThrottlingPolicy;
 use tauri::{Emitter, Manager, WebviewUrl, WebviewWindowBuilder};
+#[cfg(target_os = "macos")]
+use window_vibrancy::{
+    apply_vibrancy, clear_vibrancy, NSVisualEffectMaterial,
+};
 
 /// Global counter for generating unique window labels.
 static WINDOW_COUNTER: AtomicU64 = AtomicU64::new(1);
@@ -2358,6 +2362,78 @@ fn build_init_script(local_origin: &str) -> String {
     init_script
 }
 
+fn parse_theme_override(theme_mode: Option<&str>, theme_variant: Option<&str>) -> Option<tauri::Theme> {
+    match theme_mode.map(str::trim) {
+        Some("system") => None,
+        Some("dark") => Some(tauri::Theme::Dark),
+        Some("light") => Some(tauri::Theme::Light),
+        _ => match theme_variant.map(str::trim) {
+            Some("dark") => Some(tauri::Theme::Dark),
+            Some("light") => Some(tauri::Theme::Light),
+            _ => None,
+        },
+    }
+}
+
+fn read_desktop_theme_override() -> Option<tauri::Theme> {
+    let settings = fs::read_to_string(settings_file_path())
+        .ok()
+        .and_then(|raw| serde_json::from_str::<serde_json::Value>(&raw).ok());
+
+    let use_system_theme = settings
+        .as_ref()
+        .and_then(|value| value.get("useSystemTheme"))
+        .and_then(|value| value.as_bool());
+
+    if matches!(use_system_theme, Some(true)) {
+        return None;
+    }
+
+    let theme_mode = settings
+        .as_ref()
+        .and_then(|value| value.get("themeMode"))
+        .and_then(|value| value.as_str());
+
+    let theme_variant = settings
+        .as_ref()
+        .and_then(|value| value.get("themeVariant"))
+        .and_then(|value| value.as_str());
+
+    parse_theme_override(theme_mode, theme_variant)
+}
+
+#[cfg(target_os = "macos")]
+fn apply_macos_window_vibrancy(window: &tauri::WebviewWindow) {
+    let _ = clear_vibrancy(window);
+
+    if let Err(error) = apply_vibrancy(
+        window,
+        NSVisualEffectMaterial::Sidebar,
+        None,
+        None,
+    ) {
+        log::warn!("[desktop:vibrancy] Failed to apply macOS vibrancy: {error}");
+    }
+}
+
+#[cfg(not(target_os = "macos"))]
+fn apply_macos_window_vibrancy(_window: &tauri::WebviewWindow) {}
+
+#[tauri::command]
+fn desktop_set_window_theme(
+    window: tauri::WebviewWindow,
+    theme_mode: Option<String>,
+    theme_variant: Option<String>,
+) -> Result<(), String> {
+    let override_theme = parse_theme_override(theme_mode.as_deref(), theme_variant.as_deref());
+
+    window
+        .set_theme(override_theme)
+        .map_err(|error| format!("failed to set window theme: {error}"))?;
+
+    Ok(())
+}
+
 fn is_window_state_visible(app: &tauri::AppHandle, state: &DesktopWindowState) -> bool {
     if state.width == 0 || state.height == 0 {
         return false;
@@ -2523,6 +2599,7 @@ fn create_window(
     #[cfg(target_os = "macos")]
     {
         builder = builder
+            .transparent(true)
             .hidden_title(true)
             .title_bar_style(tauri::TitleBarStyle::Overlay)
             .traffic_light_position(tauri::Position::Logical(tauri::LogicalPosition {
@@ -2532,6 +2609,8 @@ fn create_window(
     }
 
     let window = builder.build()?;
+    let _ = window.set_theme(read_desktop_theme_override());
+    apply_macos_window_vibrancy(&window);
 
     if let Some(state) = restored_state.as_ref().filter(|_| apply_restored_state) {
         if state.maximized || state.fullscreen {
@@ -2583,6 +2662,7 @@ fn create_startup_window(app: &tauri::AppHandle, restore_geometry: bool) -> Resu
     #[cfg(target_os = "macos")]
     {
         builder = builder
+            .transparent(true)
             .hidden_title(true)
             .title_bar_style(tauri::TitleBarStyle::Overlay)
             .traffic_light_position(tauri::Position::Logical(tauri::LogicalPosition {
@@ -2592,6 +2672,8 @@ fn create_startup_window(app: &tauri::AppHandle, restore_geometry: bool) -> Resu
     }
 
     let window = builder.build()?;
+    let _ = window.set_theme(read_desktop_theme_override());
+    apply_macos_window_vibrancy(&window);
 
     if let Some(state) = restored_state.as_ref().filter(|_| apply_restored_state) {
         if state.maximized || state.fullscreen {
@@ -3025,6 +3107,7 @@ fn main() {
             desktop_hosts_get,
             desktop_hosts_set,
             desktop_host_probe,
+            desktop_set_window_theme,
             remote_ssh::desktop_ssh_instances_get,
             remote_ssh::desktop_ssh_instances_set,
             remote_ssh::desktop_ssh_import_hosts,