Reach shared folders from Files, not the profile menu

A folder somebody shared was reachable only by switching the whole app
to their account from the profile menu -- which nobody would think to
look in for files, and which pointed mail, calendar and contacts at them
as well. The server refused all three, so nothing leaked; it was simply
the app claiming to be somewhere it could not go.

Files now lists shared accounts itself, under "Shared with me", and opens
them in place. Only Files moves: `accountId` in its store is the account
being browsed, `ownAccountId` is the reader's, and nothing else in the
app notices.

Which accounts hold shared files cannot be worked out from capabilities.
Stalwart advertises the whole set on a shared account -- mail, calendars,
contacts, sieve, the lot, identical to a personal one, whatever was
actually shared (checked live on 0.16.19, 2026-08-27). That is why
routing alone could never have fixed this, and why the list offers every
account that is not the reader's own and lets its folders answer for
themselves. The mock's shared account now advertises the same full set,
because a mock that quietly advertised only what it shared would agree
with a fix that cannot work.

Shares also went unseen until the next sign-in. They arrive in the JMAP
session, which is fetched once and refreshed only when a session-state
change is pushed to that tab -- so a share granted while the tab was open
stayed invisible, and one removed stayed on offer. That is the two
browsers disagreeing about whether an account still existed. Opening
Files now re-reads the session, throttled, and the section header carries
a refresh for when someone is waiting on a share they have just been
promised.

The sidebar's button on Files was Compose, which wrote mail from the file
manager. It uploads.

Verified against the mock, which grew a second account to make any of
this testable: "Shared with me" lists it, opening it shows its folders
and not the reader's, the header says whose they are, "Back to my files"
returns, and the profile menu is not involved at any point.

web/src/styles/app.css

@@ -1024,3 +1024,9 @@ button.dp-open:disabled { cursor: default; opacity: .5; }
 .files-table tbody tr[draggable="true"]:active { cursor: grabbing; }
 .sidebar .nav-item[draggable="true"] { cursor: pointer; }
 .f-name .faint { flex: none; }
+
+/* The "Shared with me" header carries a refresh control, so it is a row rather
+   than the plain label the other sections use. */
+.sidebar .nav-section { display: flex; align-items: center; justify-content: space-between; gap: 8px; }
+.spin { animation: spin 1s linear infinite; }
+@media (prefers-reduced-motion: reduce) { .spin { animation: none; } }