feat: add dark mode toggle with persisted preference

[ammaarrahmed]

Summary

Add app-wide theme controls so users can choose light, dark, or system mode with persistent preference.

Scope

• Implement theme options: Light, Dark, System.
• Persist theme preference locally.
• Validate readability and contrast on primary screens in all supported modes.

Acceptance Criteria

• Theme changes apply across the app without restart.
• Theme preference persists across app relaunch.
• No major contrast regressions on core UI components.

Roadmap Link

Phase 4 -> Theme & Appearance