feat: add Opus/Sonnet model selector to "Genera preventivo" page

Adds a "Modello AI" toggle (Opus default, Sonnet option) styled like the
existing Soggetto toggle. The server action validates the keyword and maps
it to the real model id; the agent receives the resolved id and uses it in
the Anthropic messages.create call. The proposals DB record stores the
actual model used.

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

src/lib/proposal/agent.ts

@@ -11,6 +11,7 @@ type AgentInput = {
   client?: Pick<Client, "id" | "name" | "brand_name" | "brief">;
   transcripts: Pick<ClientTranscript, "title" | "content" | "call_date">[];
   offer: OfferEditorData;
+  model?: string;
 };
 
 function buildSystemPrompt(): string {
@@ -128,7 +129,7 @@ export async function generateProposalContent(
   const client = new Anthropic({ apiKey });
 
   const message = await client.messages.create({
-    model: MODEL,
+    model: input.model ?? MODEL,
     max_tokens: 8192,
     system: buildSystemPrompt(),
     messages: [{ role: "user", content: buildUserPrompt(input) }],