feat: add first-launch onboarding flow

[jatinkrmalik]

Summary

Implements a multi-step onboarding wizard for first-time users.

Changes

• New OnboardingView with 6-step wizard (welcome, permissions, model selection, hotkey config, quick test, completion)
• AppState tracks onboarding completion via UserDefaults with hasCompletedOnboarding flag
• App entry point conditionally shows onboarding window on first launch (VocaMacApp.swift)
• OnboardingWindowManager handles window lifecycle and monitoring for completion
• Unit tests for onboarding state management and step progression

Implementation Details

• Step 1: Welcome screen with app description
• Step 2: Permissions walkthrough (Microphone, Accessibility, Input Monitoring)
• Step 3: Model selection with hardware-based recommendations
• Step 4: Hotkey configuration (activation mode, key, double-tap speed)
• Step 5: Quick test recording to verify setup
• Step 6: Completion summary with next steps

All code follows VocaMac style guidelines: SwiftUI, @EnvironmentObject for state, SF Symbols for icons, proper error handling, and comprehensive doc comments.

Closes #22