Update documentation for new features and auth improvements

Expanded documentation to cover the new downloads manager, native context menu, OAuth/SSO and WebAuthn compatibility, and user agent strategy. Updated project structure and core concepts to reflect these features and clarified session and popup handling for authentication. Fixed minor typos and improved feature descriptions for clarity.

documentation/CORE_CONCEPTS.md

@@ -14,7 +14,8 @@ Electron applications have two types of processes: the **main process** and one
 
 Since the main and renderer processes are separate, they need a way to communicate. This is done through Inter-Process Communication (IPC).
 
--   **`ipcMain` and `ipcRenderer`**: Electron provides the `ipcMain` and `ipcRenderer` modules for this purpose. The main process can listen for messages from the renderer process using `ipcMain.handle`, and the renderer process can send messages using `ipcRenderer.invoke`.
+-   **`ipcMain` and `ipcRenderer`**: Electron provides the `ipcMain` and `ipcRenderer` modules for this purpose. The main process listens for messages using `ipcMain.handle`, and the renderer sends messages using `ipcRenderer.invoke`.
+	-   Nebula uses IPC for features like downloads control, bookmarks/history management, performance reports, GPU diagnostics, and window controls.
 
 -   **Context Bridge and Preload Script**: To securely expose APIs from the main process to the renderer process, Electron uses a **preload script** and the **context bridge**. The `preload.js` script runs in a special environment that has access to both the `window` object of the renderer process and Node.js APIs. The `contextBridge` is used to expose specific functions from the preload script to the renderer process, ensuring that the renderer process cannot access powerful Node.js APIs directly.
 
@@ -23,3 +24,9 @@ Since the main and renderer processes are separate, they need a way to communica
 -   **Performance Monitoring**: The `performance-monitor.js` module helps track the application's performance by monitoring metrics like memory usage and page load times. This is essential for identifying and addressing performance bottlenecks.
 
 -   **GPU Configuration**: The `gpu-config.js` and `gpu-fallback.js` modules manage GPU acceleration. Electron uses the system's GPU to render content, which can significantly improve performance. However, GPU drivers can sometimes be a source of instability. These modules allow Nebula to check the GPU status and apply fallbacks (like disabling hardware acceleration) if issues are detected, ensuring a more stable experience.
+
+### Authentication & User Agent Strategy
+
+-   **Auth Flow Compatibility**: Nebula allows popup windows for http/https to preserve OAuth/SSO flows without stripping POST bodies.
+-   **WebAuthn**: Platform authenticator features are enabled where supported, with diagnostics logged to help troubleshoot availability.
+-   **User Agent**: The default Electron token is removed from the UA string to improve site compatibility while appending a `Nebula/<version>` marker.