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/Cargo.lock

@@ -635,6 +635,7 @@ dependencies = [
  "rusqlite",
  "serde",
  "serde_json",
+ "serde_yaml",
  "sha2",
  "tauri",
  "tauri-build",
@@ -4243,6 +4244,19 @@ dependencies = [
  "syn 2.0.101",
 ]
 
+[[package]]
+name = "serde_yaml"
+version = "0.9.34+deprecated"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a8b1a1a2ebf674015cc02edccce75287f1a0130d394307b36743c2f5d504b47"
+dependencies = [
+ "indexmap 2.9.0",
+ "itoa 1.0.15",
+ "ryu",
+ "serde",
+ "unsafe-libyaml",
+]
+
 [[package]]
 name = "serialize-to-javascript"
 version = "0.1.1"
@@ -5480,6 +5494,12 @@ version = "1.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f6ccf251212114b54433ec949fd6a7841275f9ada20dddd2f29e9ceea4501493"
 
+[[package]]
+name = "unsafe-libyaml"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"