[UI] Implement all app screens and navigation flows

[coderabbitai]

Overview

Build complete navigation structure and all application screens.

Current State

• Only 2 screens exist: app/_layout.tsx and app/index.tsx (landing)
• Type definitions include full navigation structure
• Expo Router configured but minimal usage

Requirements

• Implement onboarding flow: Welcome, GitHub auth, API keys, Project creation, Completion
• Main app tabs: Projects list, Agents dashboard, Chat interface, Settings
• Project screens: Detail view, File browser, Code editor, Commit history, Branch management
• Agent screens: Detail view, Task assignment, Metrics dashboard
• Settings screens: Profile, Credential management, Preferences, About/help
• Implement deep linking
• Add loading states and error screens
• Test navigation flows
• Document screen architecture

Success Criteria

• All screens from type definitions implemented
• Smooth navigation transitions
• Deep linking working
• Proper error handling

Priority: HIGH

Estimated Effort: 8-10 days

Dependencies: Component Library

Labels: ui, navigation, priority:high

Backlink: Related to PR #2