feat: refine language controls and add per-tier ChatGPT Fast mode

src/server/settings-body.ts

@@ -348,7 +348,17 @@ export function parseModelsSettingsBody(body: unknown): SettingsBodyResult<Model
 		if (!parsed.override.model) {
 			return { ok: false, error: `slot "${key}" must set a model (thinkingLevel alone cannot run — absent slot = unset)` };
 		}
-		settings[key as (typeof MODEL_SLOTS)[number]] = parsed.override;
+		const value = asRecord(raw[key])!;
+		if (value.fastMode !== undefined && typeof value.fastMode !== "boolean") {
+			return { ok: false, error: `${key}.fastMode must be a boolean` };
+		}
+		if (value.fastMode === true && parsed.override.model.provider !== "chatgpt") {
+			return { ok: false, error: "Fast mode is available only for ChatGPT models." };
+		}
+		settings[key as (typeof MODEL_SLOTS)[number]] = {
+			...parsed.override,
+			...(typeof value.fastMode === "boolean" ? { fastMode: value.fastMode } : {}),
+		};
 	}
 	return { ok: true, settings };
 }