Log crashes and tell the user instead of vanishing

[danielchalmers]

An unhandled exception used to close the clock with no message, leaving only a .NET Runtime entry in Event Viewer. Now the first one, on the UI thread or a background thread like the clock's timer:

1. Saves settings one last time, if they were loaded and can be saved.
2. Appends the error to DesktopClock.log next to the app (named after the exe, like the settings file), or to %TEMP% if that folder can't be written to. Each entry has the time, app version, Windows and .NET versions, and the full stack trace. The log starts over past 1 MB.
3. Shows one message: "DesktopClock ran into a problem and needs to close", where the log is, and where to report it.
4. Exits. Carrying on in an unknown state could leave a clock that looks fine but has stopped.

A second crash while that's happening exits right away instead of stacking dialogs. No telemetry, uploads, dumps, or auto-restart.

Tested by running the real App (including its OnStartup registration) and MainWindow in-process and forcing crashes:

Case	master	this PR
UI-thread exception	vanishes, exit 0xE0434352, no log, setting changed 0.3 s earlier lost	one message, exit 1, log written, setting kept
Exception inside the clock's timer tick	same	same as above
App folder not writable	n/a	log goes to %TEMP%, message shows that path
Existing log over 1 MB	n/a	log starts over
Second crash while the message is open	n/a	exits immediately (5.6 s run vs 12.6 s with the message held open), one message, one log entry

A normal run creates no log, and the usual smoke test passes (launch, live reload, save on exit). The message text will need keys when the localization branch lands.