Keep the empty host for a state_dir starting with //

With OmitHost set, a state_dir starting with // gave a db_url starting
file://, and SQLite read the first directory name as a host, so pixa
failed to start. OmitHost is now left off for such a directory: the URL
starts file://// and SQLite reads an empty host followed by the path.
Every other state_dir gives the same URL as before.

Model: opus-5-5

internal/config/config.go

@@ -359,11 +359,13 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 // the directory name, which SQLite decodes back, so the URL names exactly
 // this file. OmitHost writes file:/var/lib/pixa/... instead of
 // file:///var/lib/pixa/..., and keeps a relative stateDir from being read
-// as a host name.
+// as a host name. A stateDir starting with // keeps the empty host, so the
+// URL starts file://// and SQLite does not read the first directory name
+// as a host.
 func defaultDBURL(stateDir string) string {
 	dbURL := url.URL{
 		Scheme:   "file",
-		OmitHost: true,
+		OmitHost: !strings.HasPrefix(stateDir, "//"),
 		Path:     stateDir + "/state.sqlite3",
 		RawQuery: "_pragma=journal_mode(WAL)",
 	}