feat(app): one window at a time, share link as an argument

A second launch hands its request to the open window and exits: over a
Unix socket in the user's runtime directory, or a named pipe on Windows
whose name carries a random key from the user's config directory, so
another account cannot take the name first. The open window comes forward,
and a share link given as the first such argument opens the Add form with
it filled in. Setup failures are logged and the app runs without the lock.

docs/ARCHITECTURE.md

@@ -124,3 +124,4 @@ an edge case.
 | Provider artwork | cache dir, `posters/` | Yes — refetched by URL; stored scaled to 640 px |
 | Desktop-only settings | config dir, `desktop.json` | Defaults if missing — hardware decoding, cache budget; never overwritten when unparseable |
 | Window size | config dir, `window.json` | Defaults if missing or implausible |
+| Single-instance pipe key (Windows) | config dir, `instance.key` | Yes — regenerated if missing |