feat: major feature additions and improvements (#135)

## Summary

This PR includes major feature additions, bug fixes, comprehensive testing improvements, and responsive design enhancements!

## New Features ✨

### Plugin & MCP Management (#133, #134)
- **Plugin Management Panel**: Install, uninstall, enable/disable, and update plugins
- **MCP Server Management Panel**: Add/remove MCP servers, view detailed configuration
- **Marketplace Management**: Add/remove plugin marketplaces from GitHub
- Backend commands for full CLI integration (`list_plugins`, `install_plugin`, `add_mcp_server`, etc.)
- Beautiful UI with proper loading states, error handling, and theme support

### Visual Todo List Panel (#132)
- Real-time todo list display when Hikari uses the `TodoWrite` tool
- Shows pending/in-progress/completed status with visual indicators
- Progress bar and completion count
- Automatically clears on disconnect
- Theme-aware styling

### Clear Session History Button (#130)
- "Clear All Sessions" button in Session History panel
- Confirmation dialog with session count
- Keyboard support and accessibility features
- Gives users control over disk usage

### CLI Version Display (#131)
- Displays Claude CLI version in status bar
- Auto-polls every 30 seconds for updates
- Useful for debugging and feature compatibility

## Bug Fixes 🐛

### Stats Panel Scrolling (#136)
- **Fixed stats panel overflow**: Added scrollable container with `max-height` constraint
- Stats panel now scrolls when content (Tools Used, Historical Costs, Budget sections) gets too long
- Prevents content from overflowing off screen

### Agent Monitor Fixes (#122)
- **Fixed agents stuck in "running" state**: Added `SubagentStop` hook parsing
- **Fixed agents persisting after disconnect**: Call `clearConversation()` on disconnect
- **Fixed "Kill All" button**: Now properly marks all agents as errored
- **Fixed badge persisting after tab close**: Cleanup agents when conversation is deleted
- Comprehensive tests for agent lifecycle management

### Discord RPC Cleanup (#129)
- Removed file-based logging for Discord RPC
- Replaced with proper `tracing` framework usage
- Reduces disk usage and eliminates maintenance burden

### Close Modal Bug Fix (#128)
- Fixed close confirmation modal not triggering after Discord RPC refactor
- Removed frontend calls to deleted `log_discord_rpc` command
- Modal now works correctly after all operations

### Responsive Design Fixes (#118)
- Fixed top navigation icons getting cut off at small screen widths
- Fixed Connect button disappearing on narrow screens
- Fixed bottom status info (clock, CLI version) getting cut off
- Added flex-wrap and mobile-optimised layouts
- Icons-only mode on screens < 640px
- Vertical stacking on screens < 768px

## Testing Improvements 🧪

### Comprehensive Test Coverage (#114)
- **417 backend tests** (up from 408)
- **387 frontend tests** (up from 363)
- **61%+ backend code coverage**
- Added E2E integration tests for cross-platform notification commands
- New test files: `agents.test.ts`, comprehensive CLI parsing tests
- Tests for `debug_logger.rs`, `bridge_manager.rs`, `notifications.rs`
- Console mocking for cleaner test output
- Fixed flaky frontend tests

### Testing Documentation
- Updated CLAUDE.md with comprehensive testing guidelines
- Documented mocking approaches (console mocking, E2E command structure testing)
- Added step-by-step guide for adding tests to new features
- Goal to maintain ~100% test coverage documented

## Closes

Closes #114
Closes #118
Closes #122
Closes #128
Closes #129
Closes #130
Closes #131
Closes #132
Closes #133
Closes #134
Closes #136

## Technical Details

- All new backend commands properly registered in `lib.rs`
- CLI output parsing with comprehensive test coverage
- Cross-platform compatibility verified through E2E tests (Linux CI can test Windows commands)
- Theme-aware UI components using CSS variables throughout
- Proper TypeScript types for all new stores and components
- ESLint and Prettier compliant
- All Clippy warnings addressed

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #135
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src-tauri/capabilities/default.json

@@ -28,6 +28,14 @@
       "identifier": "fs:allow-write-file",
       "allow": [{ "path": "**" }]
     },
+    {
+      "identifier": "fs:scope",
+      "allow": [{ "path": "$HOME/.claude/**" }]
+    },
+    {
+      "identifier": "fs:allow-read-text-file",
+      "allow": [{ "path": "$HOME/.claude/**" }]
+    },
     "core:window:allow-set-size",
     "core:window:allow-set-always-on-top",
     "core:window:allow-inner-size",