Changes to SSH terminal vars + add spinners

[ellie]

1. Only setup for ATUIN_OUTPUT_VARS if we are actually using it, otherwise save on latency
2. Show spinners while SSH operations are occurring

[greptile-apps]

Greptile Summary

Optimizes SSH terminal operations by only setting up ATUIN_OUTPUT_VARS when actually used in the code, reducing latency. Adds visual feedback with loading spinners during SSH connection and cancellation operations.

• Backend checks if code contains ATUIN_OUTPUT_VARS before creating remote temp files
• Frontend tracks isStarting and isStopping states for execution lifecycle
• PlayButton shows spinner and disables during SSH operations

Important Files Changed

Filename	Overview
crates/atuin-desktop-runtime/src/blocks/script.rs	Optimized SSH script execution to only create remote temp file when ATUIN_OUTPUT_VARS is used
crates/atuin-desktop-runtime/src/blocks/terminal.rs	Optimized terminal to only setup ATUIN_OUTPUT_VARS when actually referenced in code
src/lib/hooks/useDocumentBridge.ts	Added isStarting/isStopping states to track execution lifecycle transitions
src/components/runbooks/editor/blocks/Script/Script.tsx	Added spinner to PlayButton during SSH operations using isStarting/isStopping states
src/lib/blocks/terminal/component.tsx	Added spinner to PlayButton during SSH operations using isStarting/isStopping states

[BinaryMuse]

@ellie It took me a little bit to figure out why a script on a remote machine couldn't write to $ATUIN_OUTPUT_VARS. This also means you can't manually export from a terminal session. Is there a nice way we could handle this?