Fix mobile terminal scroll recovery

[ZacharyZcR]

Overview

Improve mobile terminal behavior during high-volume output by keeping bottom-following stable and giving users a quick way back to live output.

• Added: ...
• Updated: terminal output scrolling to preserve user position or stick to the bottom intentionally
• Removed: ...
• Fixed: mobile terminal sessions getting stranded away from the bottom during fast output

Changes Made

• Track xterm scroll position inside the WebView and report whether the viewport is at the bottom.
• Preserve the current viewport when the user has scrolled up to read history.
• Continue auto-following new output when the terminal was already at the bottom before the write.
• Add a native floating scroll-to-bottom button that appears only when the terminal is away from live output.
• Attempt to enable xterm WebGL rendering when available, with a quiet fallback to the existing renderer if WebGL is unsupported.

Related Issues

• Closes [BUG] Support#597

Screenshots / Demos

Not included; this needs an iOS terminal session with high-volume SSH output to verify interactively.

Validation run locally:

• npx eslint app/tabs/sessions/terminal/Terminal.tsx passes with existing warnings only.
• git diff --check passes.
• npx tsc --noEmit still fails on existing repository-wide TypeScript errors outside this change.

Checklist

• Code follows project style guidelines
• Supports mobile and desktop UI/app (if applicable)
• I have read Contributing.md