Add custom AI provider support and improve qBittorrent auth

Introduces support for custom OpenAI-compatible AI providers with configurable base URLs, including UI, backend validation, and connection testing. Enhances qBittorrent integration to support HTTP Basic Auth for reverse proxies, adds detailed debug logging, and updates documentation for both features. Also improves login page description logic and AI prompt generation for recommendations.

documentation/frontend/pages/login.md

@@ -2,7 +2,7 @@
 
 **Status:** ✅ Implemented | Real floating book covers with professional animations
 
-Stylized entry point with Plex OAuth integration, animated floating popular audiobook covers, and prominent "Login with Plex" CTA.
+Stylized entry point with Plex/Audiobookshelf authentication, animated floating popular audiobook covers, and dynamic description based on backend configuration.
 
 ## Design
 
@@ -13,6 +13,7 @@ Stylized entry point with Plex OAuth integration, animated floating popular audi
 - Multi-layer depth effect with z-index layering (0-20)
 - Dark theme optimized with glassmorphism card
 - Professional streaming service aesthetic
+- **Dynamic description** based on backend mode (Plex/Audiobookshelf) and automation status
 
 ## Authentication Flow
 
@@ -53,6 +54,18 @@ Stylized entry point with Plex OAuth integration, animated floating popular audi
 - Seed multipliers (7, 13, 17, 23, 29, 31) prevent pattern repetition
 - Math.sin() based pseudo-random for deterministic results
 
+## Dynamic Description
+
+Description text adapts to backend configuration:
+
+**Plex + Automation Enabled:** "Request audiobooks and they'll automatically download and appear in your Plex library"
+**Plex + No Automation:** "Request audiobooks for your Plex library"
+**Audiobookshelf + Automation:** "Request audiobooks and they'll automatically download and appear in your Audiobookshelf library"
+**Audiobookshelf + No Automation:** "Request audiobooks for your Audiobookshelf library"
+**Loading State:** "Your Personal Audiobook Library Manager"
+
+Automation is detected by checking for configured indexer (Prowlarr) via `/api/auth/providers` endpoint.
+
 ## State
 
 ```typescript
@@ -65,6 +78,15 @@ interface LoginPageState {
   showAdminLogin: boolean;
   adminUsername: string;
   adminPassword: string;
+  authProviders: {
+    backendMode: string;
+    providers: string[];
+    registrationEnabled: boolean;
+    hasLocalUsers: boolean;
+    oidcProviderName: string | null;
+    localLoginDisabled: boolean;
+    automationEnabled: boolean;
+  } | null;
 }
 
 interface BookCover {