refactor(auth): one account per browser session

The session held a wallet of up to six accounts, and a request picked one by a header or an
?account= parameter. It now holds one: user_id and its epoch. A sign-in replaces whoever was
signed in, sign-out and account deletion end the session, and an account refused mid-session
(revoked, suspended, deactivated) clears it. /api/me/accounts and /api/me/switch are gone, and
with them their refusal codes and the UI copy for them.

A cookie the multi-account build wrote carries no single epoch and is refused: its holder signs
in once more. A Google link finishing in a browser now signed in as someone else is refused, as
before for an account outside the wallet. A test holds that no app code reads the wallet keys or
the account header again.

frontend/src/i18n/locales/en/errors.json

@@ -24,7 +24,6 @@
     "screenshot": "This screenshot doesn't exist any more",
     "link": "This share link doesn't exist any more",
     "notification": "This notification doesn't exist any more",
-    "account": "That account no longer exists",
     "collection": "This collection doesn't exist any more",
     "config_key": "There's no such setting",
     "file": "The file is no longer available",
@@ -135,9 +134,6 @@
     "gone": "This playlist no longer exists on YouTube"
   },
   "refused": {
-    "account_no_access": "That account no longer has access",
-    "account_signed_out": "That account was signed out elsewhere — sign in again",
-    "account_suspended": "That account is suspended",
     "address_has_account": "That address already has an account — delete the account instead",
     "admin_first": "Create the admin account first",
     "admin_only": "Only an admin can do that",
@@ -174,7 +170,6 @@
     "not_authenticated": "You're not signed in",
     "not_ready": "Not available yet",
     "not_redownloadable": "This download can't be downloaded again",
-    "not_your_account": "That's not an account you've signed into here",
     "only_admin_delete": "You're the only admin — make someone else admin before deleting your account",
     "own_delete": "Delete your own account in Settings → Account",
     "own_role": "You can't change your own role",

frontend/src/i18n/locales/hu/errors.json

@@ -24,7 +24,6 @@
     "screenshot": "Ez a képernyőkép már nem létezik",
     "link": "Ez a megosztási link már nem létezik",
     "notification": "Ez az értesítés már nem létezik",
-    "account": "Ez a fiók már nem létezik",
     "collection": "Ez a kollekció már nem létezik",
     "config_key": "Nincs ilyen beállítás",
     "file": "A fájl már nem érhető el",
@@ -135,9 +134,6 @@
     "gone": "Ez a lejátszási lista már nem létezik a YouTube-on"
   },
   "refused": {
-    "account_no_access": "Ennek a fióknak már nincs hozzáférése",
-    "account_signed_out": "Ezt a fiókot máshol kijelentkeztették — lépj be újra",
-    "account_suspended": "Ez a fiók fel van függesztve",
     "address_has_account": "Ehhez a címhez már tartozik fiók — azt töröld helyette",
     "admin_first": "Előbb hozd létre az admin fiókot",
     "admin_only": "Ezt csak admin teheti meg",
@@ -174,7 +170,6 @@
     "not_authenticated": "Nem vagy bejelentkezve",
     "not_ready": "Még nem érhető el",
     "not_redownloadable": "Ez a letöltés nem tölthető le újra",
-    "not_your_account": "Ebbe a fiókba itt nem léptél be",
     "only_admin_delete": "Te vagy az egyetlen admin — a fiókod törlése előtt tegyél meg valaki mást adminnak",
     "own_delete": "A saját fiókodat a Beállítások → Fiók alatt törölheted",
     "own_role": "A saját szerepkörödet nem módosíthatod",