Problem

`_initDatabaseDesktop` distinguishes between cross-process (Linux: signal the running instance and exit) and intra-process (Windows: `Store.attach`) collisions when `openStore()` fails. but on Linux, when an isolate or another entry point in the same process opens the store first, the main thread's `openStore()` raises:

```
another store is still open using the same path
```

…which currently falls into the `Platform.isLinux` branch and silently `exit(0)`s the app — even though there's no second process to signal. `_initDatabaseMobile` already handles this case correctly with `Store.isOpen()` + attach; desktop didn't.

Solution

• check `Store.isOpen()` upfront and attach if true (mirroring `_initDatabaseMobile`)
• in the catch, treat the "another store is still open" exception as the intra-process signal regardless of platform, attaching to the existing store
• the `Platform.isLinux` exit path now only fires for genuine cross-process collisions (i.e. a different running instance owns the LMDB lock)

Testing

• Linux desktop: the exit-on-isolate-collision regression is gone; main thread now attaches cleanly
• second-instance launch on Linux still triggers the signal-and-exit path correctly (the foreground hand-off works as before)
• Windows: behaviour unchanged — same exception string, now matched by the platform-agnostic branch instead of the `Platform.isWindows` one