Terminal tab keys use the format "peerId_terminalId". The previous code
used split('_')[0] or startsWith('$peerId_') to extract the peerId,
which breaks when the peerId itself contains underscores.
This can happen in two scenarios:
- Hostname-based ID: when OPTION_ALLOW_HOSTNAME_AS_ID is enabled, the
peerId is derived from the system hostname, which commonly contains
underscores (e.g. "my_dev_machine").
- Custom ID: the validation regex ^[a-zA-Z][\w-]{5,15}$ allows
underscores since \w matches [a-zA-Z0-9_], so IDs like "my_dev_01"
are valid.
Fix all three parsing sites in terminal_tab_page.dart to use
lastIndexOf('_'), which is safe because terminalId is always a plain
integer with no underscores.
