Keep the empty host when the db_url path starts with //

defaultDBURL kept the empty host only when state_dir started with //,
but SQLite reads the path written into the URL, state_dir followed by
/state.sqlite3. For a state_dir of / that path is //state.sqlite3, the
URL started file:// and SQLite read state.sqlite3 as a host, so pixa
failed to start. The decision now looks at that path: a state_dir of
/ gives file:////state.sqlite3. Every other state_dir gives the same
URL as before.

Model: opus-5-5

internal/config/config.go

@@ -359,14 +359,18 @@ 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. 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.
+// as a host name. The path written into the URL is stateDir followed by
+// /state.sqlite3; when it starts with //, as it does for a stateDir of /
+// or one starting with //, the empty host is kept, so the URL starts
+// file://// and SQLite does not read the first name in the path as a
+// host.
 func defaultDBURL(stateDir string) string {
+	dbPath := stateDir + "/state.sqlite3"
+
 	dbURL := url.URL{
 		Scheme:   "file",
-		OmitHost: !strings.HasPrefix(stateDir, "//"),
-		Path:     stateDir + "/state.sqlite3",
+		OmitHost: !strings.HasPrefix(dbPath, "//"),
+		Path:     dbPath,
 		RawQuery: "_pragma=journal_mode(WAL)",
 	}