refactor(config): consolidate NEXTCLOUD_PUBLIC_ISSUER_URL through Settings

Lift NEXTCLOUD_PUBLIC_ISSUER_URL out of raw os.getenv reads into
Settings.nextcloud_public_issuer_url across all 8 production call sites
(app.py x2, oauth_routes.py x2, browser_oauth_routes.py,
provision_routes.py, userinfo_routes.py, elicitation.py). cli.py
remains the env-write source so the existing config-by-flag pipeline
still works.

Also addresses remaining PR #757 review nits:
- elicitation.py: align URL-present/absent wording on "open in your
  browser" so users don't try clicking in the terminal
- test_scope_authorization_stored.py: lock in the deliberately-shared
  fall-through branch with explicit declined/cancelled decorator tests
- test_elicitation.py: switch from monkeypatch.setenv to
  patch(get_settings) since Settings is now the canonical surface

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/userinfo_routes.py

@@ -474,7 +474,7 @@ async def user_info_html(request: Request) -> HTMLResponse:
     # otherwise fall back to NEXTCLOUD_HOST from settings
     settings = get_settings()
     nextcloud_host_for_links = (
-        os.getenv("NEXTCLOUD_PUBLIC_ISSUER_URL") or settings.nextcloud_host
+        settings.nextcloud_public_issuer_url or settings.nextcloud_host
     )
 
     # Build host info HTML (BasicAuth only)