feat: implement custom slash commands system

Adds a comprehensive slash command system that allows users to create and manage custom commands:

- Backend implementation in Rust for discovering, loading, and managing slash commands
- Support for both user-level (~/.claude/commands/) and project-level (.claude/commands/) commands
- YAML frontmatter support for command metadata (description, allowed-tools)
- Command namespacing with directory structure (e.g., /namespace:command)
- Detection of special features: bash commands (\!), file references (@), and arguments ($ARGUMENTS)

Frontend enhancements:
- SlashCommandPicker component with autocomplete UI and keyboard navigation
- SlashCommandsManager component for CRUD operations on commands
- Integration with FloatingPromptInput to trigger picker on "/" input
- Visual indicators for command features (bash, files, arguments)
- Grouped display by namespace with search functionality

API additions:
- slash_commands_list: Discover all available commands
- slash_command_get: Retrieve specific command by ID
- slash_command_save: Create or update commands
- slash_command_delete: Remove commands

This implementation provides a foundation for users to create reusable command templates and workflows. Commands are stored as markdown files with optional YAML frontmatter for metadata.

Addresses #127 and #134

src-tauri/src/commands/mod.rs

@@ -3,3 +3,4 @@ pub mod claude;
 pub mod mcp;
 pub mod usage;
 pub mod storage;
+pub mod slash_commands;