Modernize code style and unify mobile/desktop frontend

[MatteoFormentin]

Backend:

• Eliminate implicit globals (db, logger, moment, passport, etc.) across
  the whole src tree in favor of explicit require()s per module
• Break the database.js/query-module circular dependency by introducing
  database/connection.js as the single source for the raw mysql connection
• Replace var with const/let, == with ===, string concatenation with
  template literals, and callback-style code with async/await where used
• Fix bugs surfaced by removing globals and modernizing: broken
  express-validator v7 subpath imports, passport 0.7 req.logout() missing
  callback, undefined id refs in log messages, an always-true array
  comparison in the firmware update check, a dead last_update branch,
  and a fire-before-await ordering bug in firmware update deletion
• Pin package.json dependencies (was entirely "latest") and drop unused
  packages (async, bluebird, body-parser, debug, dotenv, morgan)

Frontend:

• Remove express-device and the server-side mobile/desktop template
  split; delete views/mobile and unify each page into a single
  responsive template
• Rewrite style.css as a small design-token based system (colors,
  cards, nav, forms) applied via layout.pug so it cascades across
  every page
• Redesign the home page as a responsive station card grid instead of
  a fixed desktop table duplicated for mobile

Verified end-to-end against a real MySQL instance: schema init, admin
seed, login/logout, station CRUD, sensor ingestion, history/map/config
pages.