feat(frontend): provider OpenCode dynamique + saisie provider personnalisé (#92)

Le picker de provider OpenCode du first-run wizard consomme le
catalogue dynamique exposé par le backend et propose une option
« provider personnalisé » avec un formulaire de saisie libre
(id + clé API) quand le provider souhaité n'y figure pas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/domain/index.ts

@@ -1015,6 +1015,26 @@ export interface OpenCodeProviderConfig {
   model: string;
   /** Opaque reference to the sealed API key; never the literal key. */
   apiKeyRef: string;
+  /**
+   * Additive config for a **custom** provider (endpoint outside the OpenCode
+   * registry, ticket #92). Absent = known catalogue provider (unchanged
+   * behaviour); present = arbitrary OpenAI-compatible endpoint.
+   */
+  custom?: CustomProviderConfig;
+}
+
+/**
+ * Config for a custom OpenCode provider (mirror of the backend
+ * `CustomProviderConfig`, camelCase wire format), carried by
+ * {@link OpenCodeProviderConfig.custom}.
+ */
+export interface CustomProviderConfig {
+  /** AI SDK npm package used to talk to this provider (e.g. `"@ai-sdk/openai-compatible"`). */
+  npm: string;
+  /** Base URL of the OpenAI-compatible endpoint. */
+  baseUrl: string;
+  /** Optional display label for the model (defaults to the model id). */
+  displayName?: string;
 }
 
 /**