feat(FR-3205): redesign custom deployment Start Command with shell/exec modes

Wrap the command/config-file region in one collapsible card, move the
command|config-file segment into the card header, and add a Basic/Advanced
switch that reveals Execution (Shell|Exec) + Shell selector.

Switch the write/prefill path to the new 26.7.0 command/shell GraphQL fields
(raw string sent to backend; no client-side tokenization), with version-gated
fallback to the deprecated startCommand list on older managers.

Shell mode: multi-line input, shell operators work.
Exec mode: single-line input (Enter blocked), warns on shell operators, shell
sent as null so the backend shlex-splits and exec's directly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: agatha197