Add OpenCloud connection provider

[palchrb]

What

Adds OpenCloud (https://opencloud.eu) as a cloud provider in the connect
dialog — a data-driven entry under common/loginpage/providers/opencloud/:
config.json plus list/button icons for both themes (monochrome, derived from
OpenCloud's official mark). Five new files, no code or build changes — the
providers directory is enumerated at app startup.

This answers the community-integration invitation in Euro-Office/core#77:
OpenCloud pairs naturally with the Euro-Office document server, since OpenCloud
serves its web office through WOPI natively — no platform connector app needed.

Design notes

• check.url is status.php: OpenCloud serves the ownCloud-compatible status
  endpoint, so reachability probing works like the ownCloud provider's.
• editorPage is regex:\/external-[a-z0-9-]+ rather than a hardcoded app
  name: OpenCloud opens WOPI apps on /external-<app> URLs where <app> is
  the server's configured COLLABORATION_APP_NAME — deployment configuration,
  not a constant.
• The existing ownCloud provider entry cannot serve OpenCloud: its
  editorPage (/apps/onlyoffice/) is the classic connector-app path and
  never matches, so documents are never promoted to editor tabs.
• No editorFrameSize — the default behaves correctly with OpenCloud's
  editor page.

How this was tested

Against a real OpenCloud 7.5 instance (external reverse proxy, external OIDC
IdP) with the provider directory dropped into an installed DesktopEditors
build's providers/ folder (runtime enumeration — no rebuild needed):

• provider appears in the connect dialog with icons in both themes
• OIDC login (redirect flow) works in the embedded shell
• opening a document promotes it to an editor tab via the /external-* match;
  editing, saving and co-editing with a browser user work
• recent files track the portal and reopen correctly
• together with a small OpenCloud web extension announcing portal:login
  (proposed at opencloud-eu/web-extensions#), the portal appears and
  persists in the Clouds list

Ref: Euro-Office/core#77