feat: add project settings page with tabbed layout

- Create SettingsTabLayout component with sidebar navigation
- Create ProjectSettingsPage with General settings tab
- Create RepositoriesSettingsTab for repo management
- Add Members placeholder tab
- Update router with settings routes
- Add CSS styles for settings layout
- Navigate to /projects/:id/settings from workspace header

openspec/changes/workspace-visual-overhaul/proposal.md

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+# Workspace Visual Overhaul
+
+## Problem
+
+The current repository workspace layout has usability issues:
+
+1. **Scattered navigation**: Git toolbar buttons, repository management, and project actions are in different places
+2. **No centralized project settings**: Managing repositories requires leaving the workspace
+3. **Cluttered sidebar**: Too many elements competing for attention
+4. **Inconsistent placement**: Actions aren't where users expect them (GitHub-style top-right)
+
+## Solution
+
+Redesign the workspace layout to follow GitHub-style conventions:
+
+1. **Header actions on the right**: All primary actions (History, Settings, etc.) in the top-right
+2. **Project settings page**: Centralized location for all project configuration
+3. **Repository management in settings**: Move "Manage Repositories" into project settings
+4. **Cleaner sidebar**: Focus on file tree and branch switching
+5. **Consistent navigation patterns**: Match familiar GitHub/GitLab layouts
+
+## Benefits
+
+- **Familiar interface**: Users already know GitHub's layout
+- **Cleaner workspace**: Less clutter in the main view
+- **Centralized settings**: All project config in one place
+- **Better hierarchy**: Clear separation between workspace and management
+
+## Scope
+
+### What's changing:
+- Workspace header layout (buttons moved to right)
+- New Project Settings page
+- Repository management moved to settings
+- Sidebar simplification
+- Navigation restructuring
+
+### What's staying:
+- File browser functionality
+- Git toolbar in workspace
+- File viewer/editor
+- Branch selector
+
+## Design Goals
+
+1. **GitHub-like header**: Project name left, actions right
+2. **Settings as primary action**: Gear icon in header
+3. **Streamlined sidebar**: Only file tree + branch selector
+4. **Settings tabs**: General, Repositories, Members (future), etc.