fix(auth): a tab acts only as the account it was built for, and the review's other findings

A tab names the account it was claimed for on every request, beacon and upload
(X-Siftlode-Expect-Account); current_user and /auth/logout refuse a mismatch with
401 account_changed and keep the session, so a tab left showing the account
signed in before reloads instead of writing to the new one.

/auth/login clears a connect marker left by an abandoned connect, and a link
completes only on a session that still holds. The prefs merge reads the row
under a lock. A tab's sessionStorage records its own owner. A page the account
loses is given up, address bar and remembered page included. The panel layouts
are preferences; the storage contract also refuses a key handed to the Storage
API or to usePref outside its kind. A refused sign-in as someone else and a
failed sign-out say so. The chromium-only e2e specs are in the gate register,
the member walk waits for its requests, and devices.spec checks the save itself.

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

@@ -55,6 +55,7 @@
   },
   "writeFailed": {
     "hideChannel": "Couldn't change whether the channel is hidden",
+    "signOut": "Couldn't sign you out",
     "blockChannel": "Couldn't change the channel's block",
     "unblockChannel": "Couldn't unblock the channel",
     "channelSettings": "Couldn't save the channel's settings",
@@ -134,6 +135,7 @@
     "gone": "This playlist no longer exists on YouTube"
   },
   "refused": {
+    "account_changed": "This tab was showing another account than the one signed in now, so it reloads as that one",
     "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",

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

@@ -31,6 +31,7 @@
     "about": "About",
     "signOut": "Sign out",
     "signInAsOther": "Sign in as someone else",
+    "switchNotApproved": "That Google account isn't approved for this instance yet, so you're still signed in as before.",
     "googleAddress": "Google account: {{email}}",
     "activity": "Activity"
   },

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

@@ -55,6 +55,7 @@
   },
   "writeFailed": {
     "hideChannel": "Nem sikerült módosítani a csatorna elrejtését",
+    "signOut": "Nem sikerült kijelentkeztetni",
     "blockChannel": "Nem sikerült módosítani a csatorna tiltását",
     "unblockChannel": "Nem sikerült feloldani a csatorna tiltását",
     "channelSettings": "Nem sikerült menteni a csatorna beállításait",
@@ -134,6 +135,7 @@
     "gone": "Ez a lejátszási lista már nem létezik a YouTube-on"
   },
   "refused": {
+    "account_changed": "Ez a fül más fiókot mutatott, mint ami most be van lépve, ezért újratölt azzal",
     "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",

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

@@ -31,6 +31,7 @@
     "about": "Névjegy",
     "signOut": "Kijelentkezés",
     "signInAsOther": "Belépés más fiókkal",
+    "switchNotApproved": "Ez a Google-fiók még nincs jóváhagyva ezen a példányon, ezért továbbra is az eddigi fiókkal vagy belépve.",
     "googleAddress": "Google-fiók: {{email}}",
     "activity": "Napló"
   },