fix: recompute terminal layout on mobile breakpoint changes

[web3dev1337]

Summary

• Track mobile/desktop viewport mode transitions in the orchestrator client.
• Normalize sidebar state when crossing the mobile breakpoint (clear transient mobile sidebar-open state and restore desktop preference).
• Recompute terminal grid layout when the viewport crosses mobile/desktop boundaries so returning to desktop restores the desktop terminal column layout.

Testing

• node --check client/app.js
• node --check client/terminal.js

[web3dev1337]

Reviewed in the open-PR sweep. Found and fixed a real gap: the debounced resize callback compared two point-in-time snapshots of the layout mode, but resize events come in bursts and only the last event's callback runs — a breakpoint crossing mid-burst (continuous drag through 768px, orientation change) was silently missed, which is the exact scenario the PR targets. The crossing is now latched in syncDesktopMobileLayoutState and consumed by the debounced callback. Also noting for the record: ~70% of this diff is a second, undisclosed feature (terminal history preload from /api/sessions/:id/log + scroll-listener consolidation + 100ms→180ms scroll-settle change). It was verified synchronization-safe and merges cleanly against current main, but the PR description doesn't mention it — worth updating the description before merge.