fix(cards): restore brand accent on done-state assistant reply

[esengine]

Summary

PR #126 (0.23.0) shipped a brand-toned borderLeft accent on done assistant Markdown — explicitly responding to lamyc's RFC #20 ask: "Model response readability — A background highlight on responses (instead of just text color) would make long replies much easier to follow."

PR #136 (0.23.1) refactored reasoning + streaming cards onto the shared CardBox primitive. In that refactor, the done-state color was changed from the brand tone to FG.faint (#484f58, dim grey). The bar is still technically there, but it's so muted it doesn't deliver the contrast that #126 was designed to provide — long replies blend into surrounding scrollback again.

This restores the done state to CARD.streaming.color (= TONE.brand, #79c0ff), matching the streaming state. The bar now reads continuously across the streaming → done flip.

Aborted state stays on FG.faint (correctly de-emphasized — that part of #136 was right).

Diff

One line in src/cli/ui/cards/StreamingCard.tsx:

   if (card.done && !card.aborted) {
     return (
-      <CardBox color={FG.faint}>
+      <CardBox color={CARD.streaming.color}>

Test plan

• npm run verify — 1824/1824 tests pass
• Visual check in actual TUI — confirm the done-state bar is visible against scrollback (this is the whole point)
• CI green

Out of scope

• Migrating the other 15 card types onto CardBox (separate change — currently only StreamingCard and ReasoningCard use it)
• True full-width background highlight (lamyc's literal ask) — would require per-line <Text backgroundColor> wrapping, separate RFC

Refs

• RFC discussion RFC · CLI / TUI design + interaction — what works, what is missing, what would you change? #20 (lamyc's readability ask)
• PR feat(cards): left accent bar on done assistant replies #126 — original feat that shipped the brand accent
• PR fix(tui): redesign reasoning + streaming cards #136 — refactor that accidentally toned it down