fix: Quit & Reopen still silently ate the relaunch (attempt 3)

[Alfredoalv13]

Summary

• fix: relaunch button could silently quit without reopening #13 and fix: Quit & Reopen could silently quit without ever reopening #15 both attempted to fix "Quit & Reopen" silently quitting without reopening. Verified directly with temporary debug logging this session that fix: Quit & Reopen could silently quit without ever reopening #15's fix (release the single-instance lock, then spawn `open`, then `app.exit()`) still didn't work.
• Root cause, confirmed via logging: macOS's `open` command, when the target app bundle is already running as an OS process, brings the existing instance to the front rather than launching a second one - this has nothing to do with Electron's own `requestSingleInstanceLock()`, which fix: Quit & Reopen could silently quit without ever reopening #15 correctly fixed but wasn't the whole story. Since this process is still alive (mid-teardown) at the moment `open` runs, open just re-activates it, and once it then calls `app.exit()`, nothing is left running at all.
• Fix: since nothing can run in this process after `app.exit()`, the wait for "is the old process actually gone yet" has to happen in a separate, already-spawned, detached process. A tiny polling shell script (`while kill -0 ; do sleep 0.05; done; open `) does that.
• Verified directly, 3/3 consecutive clicks, that the app reliably reopens every time with this approach (previous two attempts failed on the very first live test).

Test plan

• `tsc --noEmit` clean
• `npm test` - 94/94 files, 819/819 tests passing
• Manual: verified directly with temporary debug logging (removed before this PR) that this reopens the app reliably across repeated clicks