Hide identities from the compose picker

An account using a unique address per service, on a server with an alias
domain, ends up with every local part twice over and a From picker
nobody can use -- while only ever sending from a handful (#73).

Identities can now be hidden from that picker, from Identities &
signatures. Hiding is presentation only: the identity still exists,
still receives, and stays listed and editable, the way an unsubscribed
folder is still a folder. That framing is mbunkus's own, and it is the
right one -- this is a UI preference, not a change to the account.

Three things it refuses to do, because a sender picker with nothing
usable in it is worse than a cluttered one:

  - it will not hide the identity a draft is already using, which would
    leave the select with no matching option and move the From line
    under the writer
  - it will not hide the default, which is what a new draft starts on;
    the button is disabled there and says why
  - if every identity is somehow hidden -- reachable only through
    settings sync, since the UI will not do it -- they are all offered
    again

The setting syncs, so the picker looks the same on every device, which
follows from DEVICE_KEYS being a list of exceptions rather than a list
of what travels.

Verified against the mock with four identities and one hidden: the
picker offers the other three, the hidden address is gone from
composing, the default's hide button is disabled, and the row says the
identity still receives.

web/src/store/settings.ts

@@ -88,6 +88,19 @@ export interface Settings {
   eventCategories: Array<{ name: string; color: string }>;
   /** Default sending identity per account (JMAP has no such flag). */
   defaultIdentityByAccount: Record<string, string>;
+  /**
+   * Identities kept out of the compose picker, by id.
+   *
+   * An account with alias domains can have every address twice over while only
+   * a handful are ever sent from, which makes the picker useless (#73). This
+   * hides them from the picker only — the identity still exists on the server,
+   * still receives, and is still listed and editable in Settings, exactly as an
+   * unsubscribed folder still exists.
+   *
+   * A flat list rather than keyed by account: identity ids are unique, and an
+   * id belonging to another account simply never matches.
+   */
+  hiddenIdentities: string[];
   /**
    * The theme the top-bar toggle goes back to from light. Remembered rather
    * than assumed, so flipping to light and back returns you to the theme you
@@ -160,6 +173,7 @@ export const DEFAULT_SETTINGS: Settings = {
     { name: "Family", color: "#9333ea" },
   ],
   defaultIdentityByAccount: {},
+  hiddenIdentities: [],
   lastDarkTheme: "ihasmail",
 };