app: leave SIGHUP alone so nohup keeps the app running (#248 follow-up)

#445 also closed the window on SIGHUP, which overrides the SIG_IGN that
`nohup fieldcraft &` sets so the app outlives its terminal. A safe API cannot
ask whether a signal was ignored at launch, so SIGHUP keeps its default.
SIGTERM and SIGINT still close the window the normal way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/crash-reports.md

@@ -78,8 +78,8 @@ diagnostics.bug_report` opens the form, `ui.prompt {"answer": "set", …}` sets
 **Check that reporting works:** with `FIELDCRAFT_CRASH_TEST=1`, `diagnostics.crash_test {mode}`
 crashes on purpose: `panic`, `recovered` or `abort` (reported at the next start).
 
-A request to terminate is no crash: on Linux and macOS, SIGTERM (`kill PID`), SIGINT (Ctrl-C)
-and SIGHUP close the window as **Close** does (unsaved work is autosaved for recovery, the
+A request to terminate is no crash: on Linux and macOS, SIGTERM (`kill PID`) and SIGINT (Ctrl-C)
+close the window as **Close** does (unsaved work is autosaved for recovery, the
 journal ends, no report). A second signal ends the app at once.
 
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