[stable-34.0] Remove QtWebEngine - #10278

Drop the legacy embedded login flow (Flow v1 / WebViewFlow) and every
piece of build, packaging, and signing infrastructure that existed to
support it. The modern OAuth flow (Flow2, used by Nextcloud server
>= 16.0.0) already drives login through the system browser and remains
the only authentication path.

User-visible consequence: Nextcloud servers older than 16.0.0, and any
server advertising the legacy globalscale.desktoplogin=1 capability,
can no longer be added through an in-app embedded browser. Modern
servers are unaffected.

Changes:
- delete src/gui/wizard/webview{,page}.{h,cpp,ui,ui.license}
- drop BUILD_WITH_WEBENGINE option, find_package calls, and
  target_link_libraries entries from CMake; drop the cmakedefine in
  config.h.in
- remove DetermineAuthTypeJob's WebViewFlow enum value and the
  third (OldFlow) capability probe; the job now only runs the GET and
  PROPFIND probes
- remove ConfigFile::forceLoginV2 / setForceLoginV2 and the
  forceLoginV2 settings key
- drop the useFlow2 boolean machinery: WebFlowCredentialsDialog now
  always hosts Flow2AuthWidget, and OwncloudWizard / OwncloudSetupPage
  no longer thread a useFlow2 flag
- drop Page_WebView, WebViewPage, the welcomepage WebEngine branch
  (Create Account now always opens the system browser), and related
  #ifdef WITH_WEBENGINE blocks
- remove --without-webengine flag and buildWithWebEngine craft option
  from mac-crafter
- remove signQtWebEngineProcessApp helper from mac-crafter's Signer
- remove the WiX/MSI rule that stripped Qt6WebEngineCore.pdb
- remove qt5.qtwebengine (and the now-dead isARM helper) from the Nix
  flake

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
Signed-off-by: Iva Horn <[email protected]>