Shared calendars in the calendar, and no more account switcher

Three things from using it on two real accounts.

A calendar shared with you never appeared. Nothing was wrong with the
share -- the calendar had nowhere to be shown. Calendars loaded from one
account and one only, so the sharer's were reachable solely by switching
the whole app to their account, which is the door being closed below.
They now sit under "Shared with me" beside the reader's own, in their own
colour, with their events in the grid and a click to hide them like any
other calendar.

Their events go through `instancesIn`, the one funnel every view already
reads, so month, week, day and agenda got them without being touched.
Events and calendars from another account are keyed by account as well as
id, and hiding one is remembered under the same key: an id means nothing
outside the account holding it, and two accounts sharing an id is
ordinary rather than unlucky.

An account that shared nothing was listed in Files as though it had.
Every non-personal account was offered on the reasoning that its folders
could speak for themselves -- but an account whose *calendar* was shared
has no folders to speak with, and appeared as an invitation to open an
empty pane. Each is now asked for one file before being listed, and
silence is taken for an answer.

And the account switcher is gone from the profile menu. It existed to
reach what other people shared and was the wrong door: it moved the whole
app to somebody else's account, and since Stalwart advertises every
capability on a shared account, mail, calendar and contacts went with it
and were refused. Everything it was for is now in the module the share
belongs to, found without anyone needing to know an account was involved.

What this does not prove is that Stalwart delivers a calendar share at
all. The mock says the client handles one, which is the half that was
missing; whether the server behaves like address books, which work, or
like mail folders, which do not, needs the two accounts again.

web/src/store/files.ts

@@ -133,14 +133,27 @@ export const useFiles = create<FilesState>((set, get) => ({
      * Which accounts hold shared files cannot be worked out from capabilities:
      * Stalwart advertises the whole set on a shared account -- mail, calendars,
      * contacts and the rest -- identical to a personal one, whatever was
-     * actually shared (checked on 0.16.19, 2026-08-27). So every account that
-     * is not the reader's own is offered, and what it really holds is settled
-     * by asking it for its folders and showing what comes back.
+     * actually shared (checked on 0.16.19, 2026-08-27). So each one is asked
+     * for its files, and only the ones that answer with any are listed.
+     *
+     * Listing them all and letting the folders speak for themselves was the
+     * first attempt, and it put an account holding nothing at all under
+     * "Shared with me" -- an invitation to open an empty pane, offered by an
+     * account whose calendar or contacts were the thing actually shared. An
+     * account that shares no files does not belong in a list of shared files.
      */
     const s = session.session;
-    const sharedAccounts = Object.entries(s?.accounts ?? {})
-      .filter(([, a]) => a.isPersonal === false)
-      .map(([id, a]) => ({ id, name: a.name }));
+    const candidates = Object.entries(s?.accounts ?? {}).filter(([, a]) => a.isPersonal === false);
+    const sharedAccounts: SharedAccount[] = [];
+    for (const [id, a] of candidates) {
+      try {
+        const res = await client.call<QueryResponse>("FileNode/query", { accountId: id, limit: 1 });
+        if (res.ids.length) sharedAccounts.push({ id, name: a.name });
+      } catch {
+        // Refused means nothing here is ours to see, which is the same answer.
+        continue;
+      }
+    }
     // Stay where the reader is if they are reading a share that still exists.
     const browsing = get().accountId;
     const keep = browsing && (browsing === ownAccountId || sharedAccounts.some((a) => a.id === browsing));