fix(terminal): prevent crash in headless/Docker environments (#108)

[Lexus2016]

Closes #108

Summary

When attempting to open a graphical terminal or delegate in a headless Linux / Docker environment, Claude Code Studio threw an uncaughtException:

Error: Executable not found in $PATH: "gnome-terminal"
    at spawn
    at openTerminal (/app/server.js:10865:9)

Root Cause

1. In Node.js, child_process.spawn returns a ChildProcess synchronously without throwing. The surrounding try/catch blocks in openTerminal and /api/sessions/:id/open-terminal never caught missing executables; instead, an unhandled error event was emitted on the child process, triggering an uncaughtException.
2. In Docker and headless Linux environments, neither $DISPLAY nor $WAYLAND_DISPLAY is present, making graphical terminal emulators unusable.

Changes

• Added isHeadless() and hasCommand() helper functions in delegate-terminal.js.
• openTerminal: guards against headless environments on Linux and checks command existence with hasCommand() before spawning.
• Added p.on('error', ...) listener to spawned processes to prevent unhandled ENOENT exceptions.
• /api/sessions/:id/open-terminal: guards against headless environments and safely returns { ok: false, command: fullCmd }, allowing the frontend to copy the command to clipboard.
• Added unit tests in test/delegate-terminal.test.js covering headless detection, command presence checks, and terminal candidate configurations.