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

requirements.txt

@@ -34,10 +34,8 @@ jsonschema>=4.20.0,<5.0.0
 # Requirement specifier parsing (plugin dependency satisfaction checks)
 packaging>=23.0,<27.0
 
-# Testing dependencies
-pytest>=9.0.3,<10.0.0
-pytest-cov>=4.1.0,<5.0.0
-pytest-mock>=3.11.0,<4.0.0
+# Testing dependencies live in requirements-test.txt:
+#   pip install -r requirements.txt -r requirements-test.txt
 
 # ───────────────────────────────────────────────────────────────────────
 # Optional dependencies — the code imports these inside try/except