Usage limits: give each account its own row instead of splitting one bar

[Bombatomica64]

Problem

On Usage → Limits, a provider with several accounts draws one bar per window and splits it into one column per account. With two accounts each gets half the width, with three a third, and so on. The bars get short, two accounts' remaining quota is hard to compare at a glance, and only a small label tells them apart.

This comes up whenever one provider has more than one account: two Claude instances on different orgs or logins, several Codex homes, or a CLIProxyAPI hub reporting extra accounts. With the Claude org fix in #14750, one email in two orgs also shows as two accounts, so the split gets more common.

Proposed direction

Give each account its own full-width row inside the window card, in the same order in every card of that provider:

• Each row is labelled <provider> · <instance>, for example "Claude · Work". An unnamed instance shows just the provider name.
• A provider with a single account looks the same as today.
• An account with no reading in one window keeps an empty row there, so the rows line up across cards.
• The pooled number, the reset countdowns, credits and the redeem popover stay as they are. Only the layout of the bar changes.

Scope

A prototype exists as #14751 (closed pending this discussion). It changes only apps/web/src/components/usage/UsageLimitsPooled.tsx.

Mobile (apps/mobile/src/features/usage/UsageLimitsPooled.tsx) splits its strip the same way. Should mobile follow in the same change or a later one, or stay as it is?

I'd like a maintainer's call on the direction and scope before reopening the PR.