Show usage per machine, and say why a machine has none

The server now reports limits per machine, so the screen has to as well:
one card per machine that offers a paid service, named by the machine
first, because these are one account's numbers and which account is decided
by which box ran the session.

It also has to say which of four things happened, and the reason for
splitting them shows up here rather than in the data. A machine nobody has
logged in on is working exactly as somebody set it up, so it reads as a
plain statement in ordinary text -- marking it would be the interface
nagging about a decision already made, and would dilute the marks that do
mean something. Only "couldn't reach it" and "the endpoint refused" are
coloured as faults, and they say different things because they need
different things done. The old screen drew all three in the error colour.

No machine offering a paid service is not an error either: it says so
instead of drawing nothing.

The app also stopped parsing: `available` no longer exists and
`getBoolean` on a missing key throws, so this had to land with the server
change rather than after it. An older backend sending no `state` is read as
"failed" rather than "ok", since an empty card drawn as healthy is the
worse failure.

Looked at running, against five machines: local reporting notLoggedIn with
the backend's HOME emptied, loopback-over-ssh returning real windows beside
it, an unreachable host showing ssh's own message in red, and a machine
with no Claude provider correctly absent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VETa8afmpWaYezLCqJhDB8

AGENTS.md

@@ -57,6 +57,13 @@ repo is in PLAN.md's "Backend layout" section.
   status. Status is the obvious signal and is wrong: a turn that starts and
   finishes between two polls reads as idle at both, and its own output gets
   replayed on top of itself. That bug was visible on screen as `donedone`.
+- `server/src/usage.rs` — rate-limit windows, asked **of each machine that
+  can run Claude**, not of the backend. Credentials are read through the
+  session `Transport`, so a remote setup is an ssh round trip and the local
+  one is unchanged; the HTTP call stays here. A machine with no Claude
+  provider is never asked. The four states (`ok`, `notLoggedIn`,
+  `unreachable`, `failed`) exist because a machine nobody logged in on is a
+  choice rather than a fault, and one `error` string made it look like one.
 - `server/src/models.rs` — downloaded GGUF models and the HuggingFace
   browsing behind them. Downloads are keyed by the model rather than by
   who asked, so any device can watch one; they resume through HTTP Range,