[Enhancements]: Inconsistent header typography across the system

[duhenduhen]

❗ What happened?

Headers throughout the system use different font styles, weights, and sizes.
Typography is not unified across activities and screens. This creates a visually inconsistent and looks really bad.
Also listed in #225

📱 Device Model

realme 9 Pro (oscar)

🆕 AxionOS Version

2.6-20260515

✅ Verification Checklist

• 🔍 I have searched for existing issues and found none related to this bug.
• 🔄 I have reproduced the issue after a clean install without modifying system files.
• ⚙️ I have tested with different configurations (e.g., GApps/no GApps, rooted/non-rooted).
• 📎 I have attached all necessary logs and provided reproduction steps.