chore: second-pass cleanup — dead installer branch, broken script, orphaned JS, misfiled test deps

- first_time_install.sh: removed the pip fallback branch that installed
  from requirements_web_v2.txt — a file that has not existed since the
  v2 web interface was removed (the branch always printed its own
  'not found; skipping' warning).
- scripts/remove_plugin_backups.sh deleted: its PROJECT_ROOT resolved to
  the repo's PARENT directory, and its verify_submodules() checks for
  plugin submodules from an era before plugins moved to the store — it
  could never have worked from its current location.
- plugins_manager.js: removed three functions with zero call sites
  anywhere (addKeyValuePair, formatCommit, togglePasswordVisibility) —
  verified against all templates, all JS, and the dynamic window[name]
  dispatch sites, which resolve widget-registry keys only. Also replaced
  base.html's misleading 'Legacy ... during migration' label: the file
  is deliberately loaded last and provides the LIVE implementations of
  seven window.* plugin actions that shadow same-named definitions in
  app.js/app-shell.js.
- pytest/pytest-cov/pytest-mock moved from runtime requirements.txt to
  requirements-test.txt (CI already installs both files; the installer's
  line-by-line loop simply installs three fewer packages on devices; no
  store plugin declares pytest). HOW_TO_RUN_TESTS.md updated.
- scripts/add_defaults_to_schemas.py and analyze_plugin_schemas.py
  scanned the empty legacy plugins/ dir — now scan plugin-repos/.

Verified: fresh venv installs all four requirements files with pip check
clean and pytest available; bash -n on the installer; node --check on
the JS; widget/cache guard tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

first_time_install.sh

@@ -1189,14 +1189,7 @@ else
         # real time and in order relative to this script's own echo statements
         python3 -u "$PROJECT_ROOT_DIR/scripts/install_dependencies_apt.py"
     else
-        echo "Using pip to install dependencies..."
-        if [ -f "$PROJECT_ROOT_DIR/requirements_web_v2.txt" ]; then
-            # --ignore-installed: see the Step 5 web_interface/requirements.txt
-            # install above — same apt/pip RECORD-file conflict applies here.
-            python3 -m pip install --break-system-packages --prefer-binary --ignore-installed -r requirements_web_v2.txt
-        else
-            echo "⚠ requirements_web_v2.txt not found; skipping web dependency install"
-        fi
+        echo "Web dependencies already installed from web_interface/requirements.txt in Step 5"
     fi
 
     # Create marker file to indicate dependencies are installed