refactor(claude): drop the redundant Team-tab account budget (T-158)

After live review: the shared account /usage budget read as redundant on
both the Activity and Team tabs. Usage is per-account and can't be split per
member, so one place is enough — it lives only on the Activity tab, next to
the refresh control that fetches it (T-415). Removes the Team-tab account
card, its usage prop wiring, the team.section.usage/team.usage.shared i18n
keys, and the now-obsolete team_tab test. The three-tab card facelift stays.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -22,9 +22,6 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   fleshed out to full token + syntax coverage and ships alongside a
   contrast-hardened `summer-night-hc`, both selectable in Settings → Appearance.
   (T-478)
-- **Account budget in the Team tab.** The Claude meta sidebar's Team tab now
-  shows the shared account `/usage` budget as a single labelled card (usage is
-  per-account, not per-member). (T-158)
 
 ### Changed