feat(settings): configurable shell command for new terminals

[andrewheumann]

Summary

Adds a "Shell command" field in Settings → Terminal that overrides the binary launched for new terminal sessions. Useful for users who want Collaborator to spawn fish, nushell, a non-default bash, etc. without changing their system login shell.

Empty value falls back to $SHELL (existing behavior), so this is purely additive — no effect for users who don't opt in.

Changes

• New ui.terminalCommand pref (string; empty/unset = default).
• resolveShellPath() in terminal-target.ts centralizes the lookup: config override (if it resolves on PATH) → $SHELL → platform
  default. If the configured binary isn't found, new terminals silently fall back instead of spawning-and-failing.
• Both PTY paths consume it:
  • Sidecar via the existing resolveTerminalTarget() call.
  • tmux by passing the resolved shell as the trailing arg to tmux new-session (previously tmux fell back to its default-shell,
    ignoring the config).
• Settings UI: text input above the backend radio. Commits on blur / Enter. Accepts a name on PATH (e.g. fish) or an absolute path (e.g.
  /bin/bash). Live-validates against which; a missing binary shows a red outline + inline hint so the fallback behavior is never
  surprising.
• New IPC terminal:check-shell-command backing the validation.

Test plan

• Leave the field empty → new terminals spawn $SHELL as before.
• Set to fish (on PATH) → new terminals spawn fish. Tile title shows fish.
• Set to /bin/bash → new terminals spawn bash.
• Set to /nope/does-not-exist → input shows red "Not found on PATH" hint; new terminals fall back to default (no open-then-close).
• Toggle backend between node-pty and tmux and verify the override applies in both.

[yiliush]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[andrewheumann]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.