Fix clipped terminal text in session view

[ocnc]

Summary

• add built-in terminal content insets so the first row and left edge do not clip
• keep grid sizing, selection, and edit menu positioning aligned with the inset content rect
• tighten the renderer interface to accept resolved content geometry instead of inset policy

Testing

• xcodebuild -project Spectty.xcodeproj -scheme Spectty -sdk iphonesimulator -destination 'generic/platform=iOS Simulator' build

Fixes #6