Add state persistence for user preferences

[MartinPTielemans]

Description

User preferences (selected package manager, monitoring state) are currently stored only in memory and reset on application restart. This provides a poor user experience.

Current behavior:

• Package manager selection resets to default on app restart
• Monitoring state is not persisted
• Users need to reconfigure preferences every time

Code location:

// In command.rs - only in-memory storage
static PREFERRED_PM: Mutex<String> = Mutex::new(String::new());
static CLIPBOARD_MONITORING: Mutex<bool> = Mutex::new(false);

Proposed solution:

1. Add persistent storage using Tauri's storage API or file-based config
2. Save preferences automatically when changed
3. Load preferences on startup to restore user settings
4. Add preferences validation to handle corrupted config files

Implementation suggestions:

• Use tauri-plugin-store for cross-platform storage
• Create a UserPreferences struct with serialization
• Add migration logic for future config format changes
• Add default fallbacks for missing or invalid config

Benefits:

• Better user experience with persistent settings
• Reduced friction for daily usage
• Professional application behavior
• Foundation for additional user preferences in the future