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>

web/src/styles.css

@@ -456,7 +456,8 @@ button.session-group:hover {
 }
 .topbar .grow {
   flex: 1;
-  min-width: 0;
+  /* The title stays readable: with many chips they wrap into a second row instead. */
+  min-width: min(160px, 40vw);
 }
 .topbar h1 {
   font-size: 15px;
@@ -4398,3 +4399,14 @@ span.chat-tab > button {
   text-overflow: ellipsis;
   white-space: nowrap;
 }
+
+/* Tokens and cost under each answer. */
+.turn-usage {
+  display: flex;
+  align-items: center;
+  gap: 2px;
+  margin: -6px 0 12px;
+  font-size: 11px;
+  color: var(--muted);
+  font-variant-numeric: tabular-nums;
+}