fix: terminal scroll preservation on tab switch and alt buffer exit

[JustHereToHelp]

Summary

Fixes #2384, fixes #2790

Two related scroll-to-top bugs that have been driving me crazy:

Tab switch scroll jump: Switching between tabs would reset the terminal scroll position to the top. The issue was that intermediate tab repositioning during the switch triggered resize events that reset the viewport. Fix: skip intermediate tab repositioning for already-initialized tabs in emain-window.ts, and guard handleResize to only restore scroll if dimensions actually changed.

Alt buffer exit scroll jump: Exiting a full-screen program (htop, vim, less, etc.) would scroll the terminal to the top. Fix: save the baseY scroll position via xterm.js onBufferChange, then restore it with requestAnimationFrame after the buffer switch completes. Also added an auto-scroll guard so it doesn't interfere if the user was already at the bottom.

Heads up — this fixes it in most cases but I've still seen occasional jumps in edge cases. Probably needs a couple more trigger points caught, but it's way better than before.

Test plan

• Open two terminal tabs with scrollback history
• Scroll up in one tab, switch to the other and back — position should be preserved
• Run htop or less /etc/hosts, scroll around, quit — terminal should not jump to top
• Run vim, edit something, quit — scroll should hold
• Verify normal scrolling and auto-scroll-to-bottom on new output still works

[coderabbitai]

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