Close the smaller gaps from the security review

Ask for the account password before minting an app password, and keep
sessions the proxy checks from writing the account's own registry objects,
so a session left open on someone else's machine cannot take a credential
away from it. The password is compared with what the session holds; Stalwart
is asked only when 2FA moved the session onto an app password.

Serve attachments and proxied images with no-store on a device that is not
the person's own. Give files from a winmail.dat only the types the server
would show inline. Strip direction controls from sender and attachment
names and from saved filenames.

On signing out, send what is inside its undo window, then close every
composer, so the next person to sign in does not find the last one's draft.

Group sessions by the account Stalwart names and its server, so "sign out
other sessions" also reaches a session opened as a bare or differently
cased username.

server/src/sessions.ts

@@ -13,6 +13,8 @@ export interface StoredSession {
   /** sealed JSON {username, password} */
   sealedCredentials: string;
   username: string;
+  /** Which account this is; see `accountKey`. Absent on sessions saved before it existed. */
+  account?: string;
   createdAt: number;
   lastSeenAt: number;
   expiresAt: number;
@@ -24,6 +26,8 @@ export interface StoredSession {
 export interface LiveSession {
   id: string;
   username: string;
+  /** See `accountKey`. */
+  account: string;
   /** Basic Authorization header value for upstream calls. */
   authorization: string;
   remember: boolean;
@@ -46,8 +50,27 @@ export interface SessionSummary {
   ip: string;
 }
 
+/**
+ * The key sessions are grouped by for "sign out everywhere else".
+ *
+ * Not the username as typed: Stalwart takes `[email protected]` and a bare
+ * `alice` as the same account, and a session opened either way was missing
+ * from the list and survived the sign-out. The server's own name for the
+ * account, lower-cased, and the server it lives on -- the same name on two
+ * configured servers is two accounts.
+ */
+export function accountKey(upstream: string, canonicalUsername: string): string {
+  return `${upstream}|${canonicalUsername.trim().toLowerCase()}`;
+}
+
+function accountOf(s: StoredSession): string {
+  return s.account ?? s.username.trim().toLowerCase();
+}
+
 export interface CreateSessionParams {
   username: string;
+  /** From `accountKey`; defaults to the lower-cased username. */
+  account?: string;
   password: string;
   remember: boolean;
   userAgent: string;
@@ -85,8 +108,9 @@ export interface SessionBackend {
   resolve(cookie: string | undefined): LiveSession | null;
   reseal(cookie: string | undefined, password: string): boolean;
   destroy(id: string): void;
-  destroyAllForUser(username: string, exceptId?: string): number;
-  listForUser(username: string): SessionSummary[];
+  /** `account` is an `accountKey`, as carried on `LiveSession.account`. */
+  destroyAllForUser(account: string, exceptId?: string): number;
+  listForUser(account: string): SessionSummary[];
 }
 
 const COOKIE_SEP = ".";
@@ -172,6 +196,7 @@ export class SessionStore implements SessionBackend {
       salt: salt.toString("base64"),
       sealedCredentials: seal(JSON.stringify({ u: params.username, p: params.password }), key),
       username: params.username,
+      account: params.account ?? params.username.trim().toLowerCase(),
       createdAt: now,
       lastSeenAt: now,
       expiresAt: now + ttl,
@@ -248,10 +273,10 @@ export class SessionStore implements SessionBackend {
     if (this.sessions.delete(id)) this.scheduleSave();
   }
 
-  destroyAllForUser(username: string, exceptId?: string): number {
+  destroyAllForUser(account: string, exceptId?: string): number {
     let n = 0;
     for (const [id, s] of this.sessions) {
-      if (s.username === username && id !== exceptId) {
+      if (accountOf(s) === account && id !== exceptId) {
         this.sessions.delete(id);
         n++;
       }
@@ -260,11 +285,11 @@ export class SessionStore implements SessionBackend {
     return n;
   }
 
-  listForUser(username: string): SessionSummary[] {
+  listForUser(account: string): SessionSummary[] {
     const out = [];
     for (const s of this.sessions.values()) {
-      if (s.username !== username) continue;
-      const { secretHash: _h, salt: _s, sealedCredentials: _c, ...rest } = s;
+      if (accountOf(s) !== account) continue;
+      const { secretHash: _h, salt: _s, sealedCredentials: _c, account: _a, ...rest } = s;
       out.push(rest);
     }
     return out;
@@ -274,6 +299,7 @@ export class SessionStore implements SessionBackend {
     return {
       id: s.id,
       username,
+      account: accountOf(s),
       authorization: `Basic ${Buffer.from(`${username}:${password}`, "utf8").toString("base64")}`,
       remember: s.remember,
       createdAt: s.createdAt,