feat: Gemini als Provider hinzugefügt (Free-Tier-Modelle hardcodiert)

- OpenAiService: Gemini-Provider mit Base-URL generativelanguage.googleapis.com/v1beta/openai
- GeminiApiKey aus IConfiguration (appsettings.json / appsettings.Production.json)
- Gemini Free-Tier-Modelle hardcodiert: 2.5 Pro, 2.5 Flash, 2.5 Flash Lite
- ModelsEndpoint: /api/models/gemini Endpoint
- Frontend: geminiOptgroup dynamisch befüllt
- ProviderKeys als static HashSet (für ChatService.ResolveModelId)

wwwroot/js/pages/agent-page.js

@@ -20,6 +20,7 @@ registerPage('agent', {
     const modelEl = document.getElementById('agentModel');
     const nvidiaOptgroup       = document.getElementById('nvidiaOptgroup');
     const pollinationsOptgroup = document.getElementById('pollinationsOptgroup');
+    const geminiOptgroup       = document.getElementById('geminiOptgroup');
 
     // Modelle dynamisch laden
     const loadProviderModels = async (url, prefix, optgroup) => {
@@ -42,7 +43,8 @@ registerPage('agent', {
 
     await Promise.all([
       loadProviderModels('/api/models/nvidia',       'nvidia',       nvidiaOptgroup),
-      loadProviderModels('/api/models/pollinations', 'pollinations', pollinationsOptgroup)
+      loadProviderModels('/api/models/pollinations', 'pollinations', pollinationsOptgroup),
+      loadProviderModels('/api/models/gemini',       'gemini',       geminiOptgroup)
     ]);
     const allowUserChoiceEl = document.getElementById('agentAllowUserChoice');
     const autoCommitEl = document.getElementById('agentAutoCommit');