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/i18n/en.ts

@@ -366,6 +366,7 @@ export const en = {
   "media.prev": "Previous",
   "media.next": "Next",
   "media.back": "Back to the overview",
+  "chat.turnUsageHint": "All {n} requests of this answer (tokens sent/received, cost)",
   "chat.reviewRound": "Review {round}: changes requested",
   "agents.refine": "Review loop",
   "agents.refineRounds": "Review rounds per task",