Resolve first-party widgets relative to the shell.qml directory

Quickshell.shellDir points at the directory holding the loaded
shell.qml, so the bar can find its own widgets and bar-defaults.json
without depending on OMARCHY_PATH being set in the environment. The
old behaviour silently fell back to ~/.local/share/omarchy when the
env var was unset, which is fine for the installed shell but caused
'No such file or directory' for every first-party widget when running
quickshell -p straight from the repo.

- firstPartyWidgetSource now uses Qt.resolvedUrl("widgets/<name>.qml")
  so the resolution is anchored to shell.qml itself.
- deriveOmarchyPath() in both bar/shell.qml and bar-settings/shell.qml
  walks back from Quickshell.shellDir to find the omarchy root, with
  OMARCHY_PATH still allowed as an explicit override and the install
  default as a final fallback.

default/quickshell/bar-settings/shell.qml

@@ -8,7 +8,15 @@ ShellRoot {
   id: root
 
   property string home: Quickshell.env("HOME")
-  property string omarchyPath: Quickshell.env("OMARCHY_PATH") || (home + "/.local/share/omarchy")
+  function deriveOmarchyPath() {
+    var env = Quickshell.env("OMARCHY_PATH")
+    if (env) return env
+    var dir = String(Quickshell.shellDir || "")
+    if (dir.indexOf("/default/quickshell/bar-settings") !== -1)
+      return dir.substring(0, dir.indexOf("/default/quickshell/bar-settings"))
+    return home + "/.local/share/omarchy"
+  }
+  property string omarchyPath: deriveOmarchyPath()
   readonly property string userConfigPath: home + "/.config/omarchy/bar.json"
   readonly property string defaultsPath: omarchyPath + "/default/quickshell/bar/bar-defaults.json"