Open a group's mailbox, or folders someone shared, from the account menu

A group's members, and anyone a folder was shared with, could reach that
mail over IMAP but not here: Mail read only the reader's own account.
Calendar, Contacts and Files already list other people's shares; Mail
now does too (multi-account spec, MA-A).

Such an account is listed under "Mail to show" in the account menu,
after any locked account handed to the reader, and opens in place of
the reader's own mail through the same switch AL-7 uses. Only accounts
whose Mailbox/get answers with a mailbox are offered: the server
advertises every capability on any shared account, so a colleague who
shared one calendar would otherwise appear with mail.

While one is in view:

- a bar in the palette's accent (not the locked account's red) names
  it, with "Back to my mail"; the tab title names it without a padlock;
- calendars, contacts and files stay the reader's own (viewAccountFor
  follows only a delegation now);
- writing a message uses the viewed account's identities, so a reply in
  support@ goes out as support@ and is saved in its Drafts and Sent.

Losing the account (removed from the group, share withdrawn) takes the
reader back to their own mail with the existing "You no longer have
access" notice.

New string: "Shared mailbox:" (1, English only in the other ten
catalogs). Checked in Chrome against a scratch server with a support@
group and two members. typecheck and vitest (174 files, 1533 tests)
pass.

web/src/store/session.ts

@@ -10,6 +10,7 @@ import { unsubscribeThisDevice } from "@/lib/notify/webpush";
 import { clearAllData, clearSignedInData, setDeviceTrusted } from "@/lib/storage";
 import { startIdleLogout, stopIdleLogout } from "@/lib/idleLogout";
 import { delegationOf, type Delegation } from "@/lib/delegation";
+import { findSharedMail, sharedMailCandidates, type SharedMailAccount } from "@/lib/sharedMail";
 
 export type AuthStatus = "loading" | "anonymous" | "authenticated";
 
@@ -19,12 +20,15 @@ interface SessionState {
   /** Selected mail account (defaults to primary). */
   accountId: Id | null;
   /**
-   * A locked account handed to the reader that the app shows instead of
-   * their own (inbuxa AL-7): its mail, calendar, contacts and files. The
-   * reader's settings, filters, signatures and push stay their own.
+   * Another account whose mail the app shows instead of the reader's own:
+   * a locked account handed to them (inbuxa AL-7), whose calendar, contacts
+   * and files follow, or a shared or group mailbox (MA-A), which is mail
+   * only. The reader's settings, filters, signatures and push stay their own.
    */
   viewing: Id | null;
-  /** The name of an account whose delegation ended while it was in view. */
+  /** Shared and group mailboxes the reader can open (MA-A); see lib/sharedMail. */
+  sharedMail: SharedMailAccount[];
+  /** The name of an account the reader lost while it was in view. */
   delegationEnded: string | null;
   error: string | null;
   pushConnected: boolean;
@@ -35,8 +39,10 @@ interface SessionState {
   logout(): Promise<void>;
   refresh(): Promise<void>;
   setAccount(id: Id): void;
-  /** Show a delegated account's mail, or the reader's own with null. */
+  /** Show a delegated account's or shared mailbox's mail, or the reader's own with null. */
   view(id: Id | null): void;
+  /** Finds the shared and group mailboxes the reader can open. */
+  loadSharedMail(): Promise<void>;
   clearDelegationEnded(): void;
   /** The account to read and write for a capability, honoring the account switcher. */
   accountFor(cap: string): Id | null;
@@ -58,6 +64,7 @@ export const useSession = create<SessionState>((set, get) => ({
   session: null,
   accountId: null,
   viewing: null,
+  sharedMail: [],
   delegationEnded: null,
   error: null,
   pushConnected: false,
@@ -120,7 +127,7 @@ export const useSession = create<SessionState>((set, get) => ({
     // problem next -- and the address book cached here is the same argument.
     clearSignedInData();
     client.session = null;
-    set({ status: "anonymous", session: null, accountId: null, viewing: null });
+    set({ status: "anonymous", session: null, accountId: null, viewing: null, sharedMail: [] });
   },
 
   refresh() {
@@ -130,14 +137,16 @@ export const useSession = create<SessionState>((set, get) => ({
         const s = await apiFetch<JmapSession>("/api/auth/session?refresh=1");
         client.session = s;
         setServerLocale(s.ihasmail?.userLocale);
-        // A delegation that ended takes the reader back to their own mail
+        // A delegation that ended, or a shared mailbox taken away, takes the
+        // reader back to their own mail
         const viewing = get().viewing;
-        if (viewing && !delegationOf(s, viewing)) {
+        if (viewing && !delegationOf(s, viewing) && !sharedMailCandidates(s).some((a) => a.id === viewing)) {
           const name = get().session?.accounts[viewing]?.name ?? null;
           set({ session: s, viewing: null, delegationEnded: name });
         } else {
           set({ session: s });
         }
+        void get().loadSharedMail();
       } catch {
         /* ignore */
       } finally {
@@ -152,11 +161,18 @@ export const useSession = create<SessionState>((set, get) => ({
   },
 
   view(id) {
-    if (id && !delegationOf(get().session, id)) return;
+    if (id && !delegationOf(get().session, id) && !get().sharedMail.some((a) => a.id === id)) return;
     if (id === get().viewing) return;
     set({ viewing: id });
   },
 
+  async loadSharedMail() {
+    const session = get().session;
+    const found = await findSharedMail(session);
+    // A sign-out or another account's session arrived while it was asking
+    if (get().session === session) set({ sharedMail: found });
+  },
+
   clearDelegationEnded() {
     set({ delegationEnded: null });
   },
@@ -172,7 +188,8 @@ export const useSession = create<SessionState>((set, get) => ({
   viewAccountFor(cap) {
     const { session, viewing } = get();
     const viewed = viewing ? session?.accounts[viewing] : undefined;
-    if (viewing && viewed && cap in (viewed.accountCapabilities ?? {})) return viewing;
+    // A shared or group mailbox in view is mail only (MA-A)
+    if (viewing && viewed && delegationOf(session, viewing) && cap in (viewed.accountCapabilities ?? {})) return viewing;
     return ownAccountForCapability(session, cap);
   },
 }));
@@ -194,7 +211,8 @@ function applySession(s: JmapSession, set: (p: Partial<SessionState>) => void) {
     startIdleLogout(() => void useSession.getState().logout());
   }
   const accountId = s.primaryAccounts[CAP.mail] ?? Object.keys(s.accounts)[0] ?? null;
-  set({ status: "authenticated", session: s, accountId, viewing: null, error: null });
+  set({ status: "authenticated", session: s, accountId, viewing: null, sharedMail: [], error: null });
+  void useSession.getState().loadSharedMail();
 }
 
 client.onUnauthenticated(() => {
@@ -207,7 +225,7 @@ client.onUnauthenticated(() => {
   // usual reason to be signed out here, and reloading a form someone has
   // already started typing into would throw the password away.
   void reloadIfServerRebuilt().then((reloading) => {
-    if (!reloading) useSession.setState({ status: "anonymous", session: null, accountId: null, viewing: null });
+    if (!reloading) useSession.setState({ status: "anonymous", session: null, accountId: null, viewing: null, sharedMail: [] });
   });
 });
 
@@ -220,6 +238,13 @@ export function useViewingDelegation(): Delegation | null {
   return delegationOf(session, viewing);
 }
 
+/** The shared or group mailbox in view, if one is (MA-A). */
+export function useViewingShared(): SharedMailAccount | null {
+  const viewing = useSession((s) => s.viewing);
+  const sharedMail = useSession((s) => s.sharedMail);
+  return (viewing && sharedMail.find((a) => a.id === viewing)) || null;
+}
+
 export function viewingDelegation(): Delegation | null {
   const s = useSession.getState();
   return delegationOf(s.session, s.viewing);