Update deps, docs, auth logic; add SixLabors license

- Expanded copilot-instructions.md with project/routing details and ImageSharp license handling
- Upgraded MailKit and MimeKit to v4.17.0
- Added sixlabors.lic for ImageSharp 4.x licensing
- Removed SecretManagementExtensions.cs stub
- Updated OCORE and OCORE_web submodules
- Improved unauthorized access handling in IntranetController
- Removed empty placeholder file

.github/copilot-instructions.md

@@ -1,6 +1,5 @@
 # Copilot Instructions
 
-
 ## Project Overview
 - **Fuchs Intranet** is an ASP.NET Core (.NET 10) web application — the intranet IS the entire website, served from `/`.
 - Routes: `/{fn?}/{id?}/{code?}` → `IntranetController.Index`; `/do/{fn?}/{id?}/{code?}` → `IntranetController.Do`.
@@ -30,6 +29,7 @@
 - Make use of OCORE libraries where possible, especially for common tasks such as logging, configuration management, and data access.
 - Whenever possible, prefer OCORE_web_pdf / OCORE PDF functions for PDF-related tasks over rewriting.
 - Do not use OCMS or OCMS_sharp; use only OCORE or OCORE_web.
+- For builds failing due to SixLabors.ImageSharp requiring a license (v4.0.0+), check copilot-instructions.md for the SixLabors license key/handling info before downgrading ImageSharp.
 
 ## Azure Key Vault — Secret Naming
 - Secret names must satisfy the pattern `^[0-9a-zA-Z-]+$` (alphanumerics and hyphens only; no underscores, dots, or spaces).