Value opencode Go plan requests; per-duration cache prices; usage under answers

- opencode Go reports plan requests as cost 0 with the usage in allowance;
  ModelConfig::cost values them at the model's list prices (charged amounts
  and free requests stay as reported). Rows booked at 0 are valued once on
  start (idempotent).
- opencode's price list now has per-duration cache prices for some models
  ({"30m": ..., "12h": ...}): the shortest duration is used, also in tiers.
  Saving the provider's models failed with "untagged enum ModelEntry".
- Each answer shows tokens sent/received and the cost of the turn's requests
  (usage.cost filled when messages are saved and loaded).
- The chat title keeps a minimum width in a crowded top bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/runner.rs

@@ -1113,7 +1113,8 @@ async fn stream_step(
         let mut message = Message::new(Role::Assistant, parts);
         message.id = msg_id;
         message.model = Some(model_spec.to_string());
-        message.usage = Some(usage.clone());
+        // With its cost (charged, or valued at list prices) for the line under the answer.
+        message.usage = Some(Usage { cost: Some(req.model.cost(&usage)), ..usage.clone() });
         if !message.parts.is_empty() {
             app.store.add_message(&session.id, &message)?;
             app.emit(Event::MessageAdded { session_id: session.id.clone(), message: message.clone() });