Report what a session last did and what it cost, not what this page holds

Three readings that were each a part presented as the whole.

**"just now", everywhere, after a restart.** A relaunched session took its
last-activity from the clock, so every session the backend brought back
claimed to have been active that instant. On the phone that is every row
reading "just now" and the list -- which sorts by it -- coming back in an
order that means nothing, with the conversation somebody was in the middle
of buried among sessions untouched for days. It comes from the transcript
now, in the pass `Transcript::open` already makes, which is the same
correction `last_status` got and for the same reason: a server that has just
started has been told nothing, and the file is the only thing it knows. The
test backdates a transcript by a day, so it cannot pass by the test being
fast; it fails on the old code with the clock's answer in the message.

**The token total was the newest page's.** The phone added up the
`UsageDelta`s it had received, and it opens a session on the newest page of
the transcript -- so a long conversation reported its last few turns as the
total, and a page with no turn in it reported nothing at all, since zero is
drawn as blank. That is the reading Bryan saw: no tokens, on sessions that
had certainly spent some.

The count belongs to the server, which is the only side that sees every
turn. `UsageDelta` now carries the running total beside the delta, filled in
by the pump rather than by each driver -- a driver knows what its own turn
cost and nothing else does, so a new one cannot get this wrong by leaving it
out -- and the session row reports it for a screen that has not opened the
stream yet. The phone takes the largest total it has seen instead of
accumulating, which also means paging older history cannot move it, and
leaves the seeded figure alone for transcripts recorded before the field
existed. Seeded by summing deltas at startup for exactly that reason.

**The header said the model twice and the machine backwards.** A session's
subtitle now reads `machine · provider`, in that order and with no "on"
joining them, matching the list and the usage dialog -- the "on" made it a
phrase, which works in one order and stops working the moment the same pair
is shown somewhere else. The model is gone from it: the footer's picker
already shows what the session is set to, and two places showing it meant
two things to keep in step, which disagreed for a moment on every switch
since one follows the request and the other the session's own answer.

Checked on the emulator against a twelve-turn session whose visible page
held the last six: the header reads "this machine · echo", the status row
reads "idle", and the total reads 42 tok, which is what `GET
/sessions/{id}` says rather than what the page adds up to.

server/src/session/echo.rs

@@ -446,6 +446,7 @@ impl EchoDriver {
             }
             send(Event::UsageDelta {
                 tokens: text.split_whitespace().count() as u64,
+                total: 0,
             });
             finish();
         });