app: a termination signal closes the window the normal way, not as a crash (#248)

SIGTERM, SIGINT and SIGHUP ended the process on the spot: no last autosave,
and the crash journal left behind became an unclean_exit report at the next
start. The report on #248's machine ("ended without a clean exit between
commands (last: control ui.capture)") has that exact signature and is
reproduced by kill -TERM after a capture; 500 captures of the solved
crank-mirror (50 app runs), 30 CLI snapshots and 250 MCP headless
screenshots exited cleanly. The first signal now asks the window to close;
a second ends the process with the signal's default action.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

Cargo.lock

@@ -1284,6 +1284,7 @@ dependencies = [
  "rfd",
  "rustls",
  "serde_json",
+ "signal-hook",
  "ureq",
 ]
 
@@ -3677,6 +3678,16 @@ version = "2.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8fadd59c855ef2080decdef8ff161eb6661b86933c9d82e5ba29dc602a55aba"
 
+[[package]]
+name = "signal-hook"
+version = "0.3.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d881a16cf4426aa584979d30bd82cb33429027e42122b169753d6ef1085ed6e2"
+dependencies = [
+ "libc",
+ "signal-hook-registry",
+]
+
 [[package]]
 name = "signal-hook-registry"
 version = "1.4.8"