[UI] First-launch consent dialog

[LeGeRyChEeSe]

Description

Implement the consent dialog shown once on the app's first launch.

File to create

• app/src/main/java/com/vrhub/ui/components/ConsentDialog.kt

Expected behavior

1. On first launch, display ConsentDialog (non-dismissible via back/touch outside)
2. Provide two actions: Accept and Decline
3. Persist has_seen_consent_dialog = true in SharedPreferences
4. If Accept: persist consent_enabled = true in DataStore
5. If Decline: persist consent_enabled = false (default)

UI

• Title: "Help Improve VRHub"
• Explanatory text with bullet points on collected and non-collected data
• Material3 buttons: Button (Accept) + OutlinedButton (Decline)

Integration

• Call from MainActivity.kt / MainScreen via a LaunchedEffect(Unit)
• Use ConsentPreferences to persist the choice

Reference

• docs/CLIENT_SPEC.md §2 (Consent Flow, Privacy Notice)
• docs/CLIENT_IMPLEMENTATION.md §5