fix(capabilities): grant process:allow-exit so exit buttons can quit the app

The db-version-too-new recovery screen and the config-load-failure path
both call exit() from @tauri-apps/plugin-process, but the default
capability only granted process:allow-restart. The IPC call was denied
and silently swallowed by void/await, leaving the app impossible to
quit from those screens.

src-tauri/capabilities/default.json

@@ -15,6 +15,7 @@
     "core:window:allow-is-maximized",
     "core:window:allow-close",
     "core:window:allow-set-decorations",
+    "process:allow-exit",
     "process:allow-restart",
     "dialog:default"
   ]