Authenticate at app launch

[demonkillerr]

The goal of this PR is to request for user's password only at app launch. Thereafter, the application will use cached password instead of pkexec. This change makes the user's life easy, as they do not need to authenticate multiple times for multiple operations.

The Implementation is a single-prompt authentication model at application startup using a new AuthManager singleton class. All privileged operations now use sudo -S with the cached password instead of pkexec, reducing authentication prompts and improving user experience.

Changes:

• Add AuthManager class: prompts for password once at startup, validates via sudo -S, stores securely in memory
• Integrate auth in MainWindow constructor: exits gracefully if user cancels
• Migrate PackageManager: all package operations now use sudo -S with password piping
• Migrate SettingsWidget: all 9 privilege escalation calls replaced with sudo -S
• Update UI text: remove admin privilege prompts and pkexec references
• Bump version to 0.3.0

User benefits:

• Single password prompt at startup instead of multiple prompts during operations
• Cleaner UI without repeated authentication dialogs
• Password is zeroed in memory after session ends