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/browser_oauth_routes.py

@@ -22,6 +22,7 @@ from nextcloud_mcp_server.auth.userinfo_routes import (
     _get_userinfo_endpoint,
     _query_idp_userinfo,
 )
+from nextcloud_mcp_server.config import get_settings
 
 from ..http import nextcloud_httpx_client
 
@@ -167,7 +168,7 @@ async def oauth_login(request: Request) -> RedirectResponse | JSONResponse:
             scopes += " offline_access"
 
         # Replace internal Docker hostname with public URL
-        public_issuer = os.getenv("NEXTCLOUD_PUBLIC_ISSUER_URL")
+        public_issuer = get_settings().nextcloud_public_issuer_url
         if public_issuer:
             internal_parsed = parse_url(oauth_config["nextcloud_host"])
             auth_parsed = parse_url(authorization_endpoint)