fix(web): define project_root when plugins_directory is absolute (#616)

project_root was only assigned in the relative-path branch, so an absolute
plugin_system.plugins_directory made web_interface/app.py raise NameError
at import (first use: the SchemaManager construction). Define it before the
if/else; plugins_dir resolution is unchanged.

Adds a regression test that imports the real module in a fresh interpreter
with an absolute and a relative plugins_directory.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_app_plugins_dir_resolution.py

@@ -0,0 +1,65 @@
+"""
+web_interface/app.py must start with an absolute ``plugins_directory``.
+
+``project_root`` used to be assigned only in the relative-path branch, so an
+absolute path in config.json raised NameError at module import (the
+SchemaManager construction was the first use). Each case imports the real
+module in a fresh interpreter, since app.py does its setup at import time and
+the test process may already hold a copy built from the real config.
+"""
+
+import json
+import subprocess
+import sys
+import textwrap
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+
+_IMPORT_APP = textwrap.dedent(
+    """
+    import json, sys
+    from pathlib import Path
+    sys.path.insert(0, {root!r})
+    from src.config_manager import ConfigManager
+    ConfigManager.load_config = lambda self: {config!r}
+    import web_interface.app as web_app
+    print(json.dumps({{
+        "plugins_dir": str(web_app.plugins_dir),
+        "project_root": str(web_app.project_root),
+        "schema_project_root": str(web_app.schema_manager.project_root),
+    }}))
+    """
+)
+
+
+def _import_app_with(plugins_directory):
+    config = {"plugin_system": {"plugins_directory": plugins_directory}}
+    script = _IMPORT_APP.format(root=str(PROJECT_ROOT), config=config)
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        cwd=str(PROJECT_ROOT),
+        capture_output=True,
+        text=True,
+        timeout=120,
+    )
+    assert result.returncode == 0, result.stderr
+    return json.loads(result.stdout.strip().splitlines()[-1])
+
+
+def test_absolute_plugins_directory_imports(tmp_path):
+    plugins = tmp_path / "my-plugins"
+    plugins.mkdir()
+
+    info = _import_app_with(str(plugins))
+
+    assert Path(info["plugins_dir"]) == plugins
+    assert Path(info["project_root"]) == PROJECT_ROOT
+    assert Path(info["schema_project_root"]) == PROJECT_ROOT
+
+
+def test_relative_plugins_directory_resolves_under_project_root():
+    info = _import_app_with("plugin-repos")
+
+    assert Path(info["plugins_dir"]) == PROJECT_ROOT / "plugin-repos"
+    assert Path(info["project_root"]) == PROJECT_ROOT