NTR: frontend cleanup - JS files

[cxo-jutz]

Small, behavior-preserving cleanups of the storefront JavaScript. No change to the
custom plugin base or the runtime-decoupling strategy.

Changes

• http-client: fixed a broken error callback. xhr.onerror checked
  callbackSuccess instead of callbackError, so the error handler never fired.
  Also guarded onload against a missing content-type header (getResponseHeader
  returns null, and .indexOf() on it would throw).

• Plugin base: added listener lifecycle. Introduced _addListener() (tracks each
  registered listener) and destroy() (removes them all). phone-plugin and
  subscribe-button now register through it. Shopware may re-initialize plugins on
  DOM updates (off-canvas cart, bfcache restore); without teardown this leaves
  duplicate listeners and leaks. Behavior is unchanged — the capture flag is passed
  through to both add/removeEventListener.

• js- hook decoupling. Split styling classes from JS/test hooks (matches the
  project's existing stylelint rule that bans .js- classes from styling):

  • .mollie-paypal-button (styled) → JS now selects .js-mollie-paypal-button;
    the styling class stays on the element.
  • JS-only state marker observed → js-mollie-observed.

• Removed dead IE / legacy-Edge detection. Dropped isIEBrowser, isEdgeBrowser,
  isNativeWindowsBrowser and getList from the device-detection helper, plus the
  IE-only TOS-checkbox workaround in the credit-card components (and its now-unused
  import).
  Why: IE is end-of-life, and isEdgeBrowser() matched the Edge/ UA token, which
  is legacy (EdgeHTML) Edge only — also discontinued. Modern Edge is Chromium and
  sends Edg/, so it was never matched by this code and needs none of these
  workarounds. No coverage lost for current browsers.

Notes / non-changes

• Kept native querySelector/getElementById instead of Shopware's DomAccess:
  the storefront intentionally touches Shopware only via runtime globals
  (window.PluginManager, window.csrf) and avoids build-time imports from the
  Shopware source tree, to stay decoupled across SW 6.4–6.7.