fix(startup): a launch that fails before the window says why

Reported from a Windows 10 machine: the process appears, no data folder
is created, nothing starts. There was nothing to read because the only
log OpsLog has lives inside the folder that was never created.

Three exits happen before any window or log exists, and all three were
silent. Another instance already running — correct to refuse, since two
would fight over the rig, but indistinguishable from a crash. The data
folder unwritable — it is created BESIDE the executable, so a copy
dropped into Program Files is refused by Windows outright. And wails.Run
failing, which is where a missing WebView2 runtime lands; its error went
to println, which in a GUI-subsystem program goes nowhere at all.

Each now writes to %LOCALAPPDATA%\OpsLog\startup.log — a folder Windows
guarantees the user can write to, whatever OpsLog was installed into —
and shows a message box naming the fault and what to do about it.

fatalbox_other.go

@@ -0,0 +1,6 @@
+//go:build !windows
+
+package main
+
+// fatalBox is Windows-only; elsewhere the terminal carries the message.
+func fatalBox(title, text string) { println(title + ": " + text) }