fix(node/nix): consolidate workspace lockfile + update all consumers

Consolidate per-package package-lock.json files into a single root-level
workspace lockfile.  Update all consumers:

- Nix: shared src/npmDeps/npmDepsHash in lib.nix; devshell hook stamps
  package.json paths then runs npm ci from root; individual .nix files
  use mkNpmPassthru attrs instead of per-package fetchNpmDeps.
- Python CLI: new _workspace_root() helper so _tui_need_npm_install,
  _make_tui_argv, _build_web_ui resolve lockfile/node_modules from the
  workspace root.
- Desktop: replace --force-build/mtime heuristic with content-hash build
  stamp (_compute_desktop_content_hash via pathspec).  Remove --force-build
  flag.
- Dockerfile: single root npm install; no per-directory lockfile copies.
- CI: nix-lockfile-fix and osv-scanner reference root package-lock.json;
  apps/dashboard → apps/desktop.
- Tests: new test_tui_npm_install.py; desktop stamp tests in
  test_gui_command.py; updated assertions in test_cmd_update.py,
  test_web_ui_build.py, test_dockerfile_pid1_reaping.py.
- Docs: remove --force-build from desktop flag table.

Deleted: apps/desktop/package-lock.json, ui-tui/package-lock.json,
ui-tui/packages/hermes-ink/package-lock.json, web/package-lock.json.

tests/tools/test_dockerfile_pid1_reaping.py

@@ -147,11 +147,13 @@ def test_dockerfile_installs_tui_dependencies(dockerfile_text):
     # because it's referenced as a ``file:`` workspace dependency from
     # ``ui-tui/package.json`` — copying the tree avoids npm stopping at a
     # bare ``package.json`` shell.
+    # With a single workspace root lockfile, only the root package-lock.json
+    # is copied; per-workspace lockfiles no longer exist.
     assert "ui-tui/package.json" in dockerfile_text
-    assert "ui-tui/package-lock.json" in dockerfile_text
     assert "ui-tui/packages/hermes-ink/" in dockerfile_text
+    assert "package-lock.json" in dockerfile_text
     assert any(
-        "ui-tui" in step and "npm" in step and (" install" in step or " ci" in step)
+        "npm" in step and (" install" in step or " ci" in step)
         for step in _run_steps(dockerfile_text)
     )