refactor: move webapp from /user/page to /app

Simplified the webapp routing structure by consolidating the admin UI
to a single clean endpoint.

Changes:
- Moved webapp from /user/page to /app (root of mount)
- Removed /user JSON endpoint (no longer needed)
- Updated mount point from /user to /app in app.py
- Updated all route path checks (3 locations)
- Updated OAuth redirects to point to /app
- Updated all HTMX endpoint references
- Updated documentation (ADR-007, CHANGELOG)
- Added redirect from /app to /app/ for trailing slash handling

New Route Structure:
- /app - Main webapp (HTML UI with tabs)
- /app/revoke - Revoke background access
- /app/webhooks - Webhook management UI
- /app/webhooks/enable/{preset_id} - Enable webhook preset
- /app/webhooks/disable/{preset_id} - Disable webhook preset

Breaking Change: Existing bookmarks to /user or /user/page will no longer work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/auth/userinfo_routes.py

@@ -602,7 +602,7 @@ async def user_info_html(request: Request) -> HTMLResponse:
     webhooks_tab_html = ""
     if show_webhooks_tab:
         webhooks_tab_html = """
-            <div hx-get="/user/webhooks" hx-trigger="load" hx-swap="outerHTML">
+            <div hx-get="/app/webhooks" hx-trigger="load" hx-swap="outerHTML">
                 <p style="color: #999;">Loading webhook management...</p>
             </div>
         """