Add terminal caching for faster workspace switching

[gricha]

Summary

• Cache up to 5 terminal instances using LRU eviction strategy
• Reuse cached terminal when switching back to a workspace, preserving scrollback history
• WebSocket connections are maintained per cached terminal
• Memory estimate: ~2-15MB per terminal depending on scrollback usage

Test plan

• Switch between workspace terminals and verify content is preserved
• Switch to 6+ different workspaces to verify LRU eviction works
• Verify rapid tab switching doesn't cause issues
• Confirm WebSocket reconnects properly after tab switch

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