Escape state_dir in the db_url derived from it (closes #290)

The derived db_url put state_dir into a file: URL as it was, so SQLite
decoded a %HH in the name and ended the path at a ? or #, and the
journal mode parameter was lost. It is now built with net/url, which
percent-encodes the directory name; SQLite decodes it back and opens
state.sqlite3 in that directory. OmitHost keeps the usual form
file:/var/lib/pixa/state.sqlite3 and a relative state_dir a path.

Model: opus-5-5

internal/config/config.go

@@ -338,8 +338,19 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 				settingName(keyDBURL), errValueEmpty)
 		}
 
-		// The driver sets the journal mode only through a _pragma parameter.
-		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_pragma=journal_mode(WAL)", c.StateDir)
+		// The driver sets the journal mode only through a _pragma
+		// parameter. String() percent-encodes any %, ? or # in 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
+		// state_dir from being read as a host name.
+		dbURL := url.URL{
+			Scheme:   "file",
+			OmitHost: true,
+			Path:     c.StateDir + "/state.sqlite3",
+			RawQuery: "_pragma=journal_mode(WAL)",
+		}
+		c.DBURL = dbURL.String()
 	}
 
 	if loader.err != nil {