Add inline documentation sheets to settings views

[bjorkert]

Summary

• Adds info button (ℹ) to section headers in Nightscout Settings, General Settings, Graph Settings, and Background Refresh Settings
• Tapping the button presents a contextual help sheet with documentation sourced from the LoopFollow docs site
• Uses a single .sheet(item:) per view to avoid UIKit presentation conflicts when multiple sheets exist in the same Form

Views covered

• Nightscout Settings — URL (how to find/paste your URL) and Token (what tokens are, how to create one, which role to use)
• General Settings — App Settings (badge, persistent notification), Display (stats, IFCC, small graph, color BG, keep screen active, display name), Speak BG (always vs conditional, low vs proactive low, fast drop delta)
• Graph Settings — Basal/BG Scale (min basal, min BG scale) and History (days back)
• Background Refresh — Overview of silent tune vs Bluetooth heartbeat, supported devices and battery info

[bjorkert]

Nightscout Settings:

[bjorkert]

General Settings: