resume: fall back to current-terminal when no new terminal can be spawned (#203)

When the session picker is configured for new-terminal resume (or Ctrl+Enter)
and no terminal emulator can be spawned (headless/SSH/no recognized emulator),
jcode previously dead-ended with a 'No terminal found. Resume manually:' message.

For a single jcode session (resume picker and crash-restore), fall back to
resuming in the current terminal instead. Multi-session and separate-session
flows (transfer/split/next-prompt/review) keep the manual hint by design.

Adds a reproduction test confirming spawn_command_in_new_terminal_with returns
Ok(false) when every candidate terminal is absent.