Add OpenRouter dedicated images API support

Fetches /api/v1/images/models in parallel with /models and merges results
into the model picker. Image-only models (e.g. Sourceful, Seedream, Flux
via this endpoint) were previously invisible since they don't appear in the
standard /models endpoint.

Models from the images API get usesImagesAPI=true and route through a new
generateImageAPIResponse() path in ChatViewModel that POSTs to /api/v1/images
with {model, prompt} instead of the chat completions endpoint. The response's
b64_json data is decoded and displayed via the existing GeneratedImagesView.

Cost is taken directly from the usage.cost field in the images API response
(USD per image) via a new rawCostUSD field on ChatResponse.Usage, bypassing
the token-based calculateCost() path used for chat models.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Providers/OpenRouterModels.swift

@@ -420,6 +420,67 @@ struct ToolResultMessage: Encodable {
     }
 }
 
+// MARK: - Images API — Model Discovery
+
+struct OpenRouterImageModelsResponse: Codable {
+    let data: [ImageModelData]
+
+    struct ImageModelData: Codable {
+        let id: String
+        let name: String
+        let description: String?
+        let architecture: Architecture?
+        let supportsStreaming: Bool?
+
+        struct Architecture: Codable {
+            let inputModalities: [String]?
+            let outputModalities: [String]?
+
+            enum CodingKeys: String, CodingKey {
+                case inputModalities = "input_modalities"
+                case outputModalities = "output_modalities"
+            }
+        }
+
+        enum CodingKeys: String, CodingKey {
+            case id, name, description, architecture
+            case supportsStreaming = "supports_streaming"
+        }
+    }
+}
+
+// MARK: - Images API — Generation Response
+
+struct OpenRouterImageGenerationResponse: Codable {
+    let created: Int?
+    let data: [ImageData]
+    let usage: Usage?
+
+    struct ImageData: Codable {
+        let b64Json: String
+        let mediaType: String?
+
+        enum CodingKeys: String, CodingKey {
+            case b64Json = "b64_json"
+            case mediaType = "media_type"
+        }
+    }
+
+    struct Usage: Codable {
+        let promptTokens: Int
+        let completionTokens: Int
+        let totalTokens: Int
+        let cost: Double?
+
+        enum CodingKeys: String, CodingKey {
+            case promptTokens = "prompt_tokens"
+            case completionTokens = "completion_tokens"
+            case totalTokens = "total_tokens"
+            case cost
+        }
+    }
+}
+
 // MARK: - Error Response
 
 struct OpenRouterErrorResponse: Codable {