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>

.opencode/skills/theme-system/SKILL.md

@@ -33,6 +33,43 @@ OpenChamber uses a JSON-based theme system. Themes are defined in `packages/ui/s
 - Status colors = **ONLY for actual feedback** (errors, warnings, success)
 - Input footers = `bg-transparent` on elevated background
 
+## Button Rules (MANDATORY)
+
+Use only the shared `Button` component from `packages/ui/src/components/ui/button.tsx`.
+
+- Do not create wrapper button components (for example `ButtonLarge`, `ButtonSmall`).
+- Do not hardcode button height/padding classes when a `size` variant exists.
+- Use semantic button variants consistently; avoid ad-hoc one-off button styling.
+
+### Allowed Button Variants
+
+| Variant | Use for | Token direction |
+|-------|-------|-------|
+| `default` | Primary action in a local section/dialog | `primary.*` |
+| `outline` | Secondary visible action | `surface.elevated` + `interactive.*` |
+| `secondary` | Soft secondary action | `interactive.hover` / `interactive.active` |
+| `ghost` | Low-emphasis row/toolbar action | transparent + `interactive.hover` |
+| `destructive` | Destructive actions (`Delete`, `Revert all`) | `status.error*` |
+| `link` | Rare inline text action only | text-link style |
+
+### Allowed Button Sizes
+
+| Size | Use for |
+|------|---------|
+| `xs` | Dense controls in rows/lists |
+| `sm` | Default compact action buttons |
+| `default` | Standard form/page actions |
+| `lg` | Prominent large actions |
+| `icon` | Icon-only square button |
+
+### Button Selection Quick Guide
+
+1. Main CTA in section/dialog -> `default`
+2. Side action next to CTA -> `outline`
+3. Quiet auxiliary action -> `ghost`
+4. Dangerous action -> `destructive`
+5. Tiny row action -> keep same variant, set `size="xs"`
+
 ### Never Use
 
 - Hardcoded hex colors (`#FF0000`)