Other: Fix IMAP/SMTP/Login leaks/race conditions

Depending on the timing of bridge closure, it was possible for the 
IMAP/SMTP servers to not have started serving yet. By grouping this in
a cancelable goroutine group (*xsync.Group), we mitigate this issue.

Further, depending on internet disconnection timing during user login,
it was possible for a user to be improperly logged in. This change 
fixes this and adds test coverage for it.

Lastly, depending on timing, certain background tasks (updates check,
connectivity ping) could be improperly started or closed. This change
groups them in the *xsync.Group as well to be closed properly.

internal/async/context.go

@@ -51,3 +51,32 @@ func (a *Abortable) newCancelCtx(ctx context.Context) context.Context {
 
 	return ctx
 }
+
+// RangeContext iterates over the given channel until the context is canceled or the
+// channel is closed.
+func RangeContext[T any](ctx context.Context, ch <-chan T, fn func(T)) {
+	for {
+		select {
+		case v, ok := <-ch:
+			if !ok {
+				return
+			}
+
+			fn(v)
+
+		case <-ctx.Done():
+			return
+		}
+	}
+}
+
+// ForwardContext forwards all values from the src channel to the dst channel until the
+// context is canceled or the src channel is closed.
+func ForwardContext[T any](ctx context.Context, dst chan<- T, src <-chan T) {
+	RangeContext(ctx, src, func(v T) {
+		select {
+		case dst <- v:
+		case <-ctx.Done():
+		}
+	})
+}