fix: recover plugin directory when omarchy strips __sourceDir

[CoderLambert]

Problem

After upgrading to omarchy 4.0.4, QOpen opens with "0 resources" even though
~/.config/qopen/config.json is intact and bin/qopen api catalog works
standalone.

Root cause

omarchy 4.0.4 hardened the third-party plugin API and now injects a sanitized
manifest via publicPluginManifest(), which deletes __sourceDir for
third-party plugins:

function publicPluginManifest(manifest) {
    if (!manifest) return null
    if (manifest.__isFirstParty) return manifest
    var copy = JSON.parse(JSON.stringify(manifest))
    delete copy.__sourceDir   // third-party plugins lose their own directory
    ...
}

QOpen derived its backend path from that field:

readonly property string pluginDir: (manifest && manifest.__sourceDir) || ""

so pluginDir became "", backendPath became "",
requestCatalogReload() returned early, and the catalog stayed at its
empty initial value — no error banner, just an empty list.

Fix

pluginDir now falls back to the plugin's own entry point URL, exposed by
the host-injected pluginRegistry API (file:///.../qopen.launcher/QOpen.qml
→ directory). Shells before 4.0.4 keep the existing __sourceDir path, so
behavior is unchanged there.

Verification

• python -m json.tool manifest.json OK
• python -m py_compile bin/qopen OK
• python -m unittest discover -s tests -v — 47 tests OK (incl. new
  source-level regression test for the fallback)
• omarchy plugin validate . — exit 0
• qmlcachegen --only-bytecode on all five QML files OK
• git diff --check clean
• Runtime acceptance on omarchy 4.0.4 still to be performed manually

🤖 Generated with Claude Code