Adds Services/SelfUpdateService.cs implementing the Chrome-style pattern:
1. Download new exe to "<current>.new"
2. Rename running exe to "<current>.old" (NTFS allows MoveFile on a
running exe -- it resolves processes by handle, not by path)
3. Rename ".new" to canonical path
4. Spawn the new exe with our argv
5. Environment.Exit(0)
6. Next start of the new exe runs CleanupAfterUpdate() in App.OFIC()
which deletes the leftover ".old"
UI: the existing "Download" banner button is now "Install update". Click
runs the self-update flow with a percent/MB progress label; any failure
(network, AV write-block, dir not writable) falls back to opening the
URL in the browser so users always have a path forward.
Bumps to 0.4.7. Also fixes a stale fallback URL that pointed at
sijbers.uk/pack/... -- now correctly points at bns.sijbers.uk/launcher/.
Matt Sijbers