feat(auth): N3 — server-side multi-session account switch

Track every account that completes OAuth in a browser session (session 'account_ids',
~14-day cookie), so the user can switch between them without another Google round-trip.
New GET /api/me/accounts (switchable accounts, active flagged) and POST /api/me/switch
(guarded: target must be in the session list — proof it signed in here — and re-checked
against is_allowed in case the invite was revoked). Logout now signs out the active
account and falls back to the most recent remaining one, else clears the session. UI: the
account popover lists the other signed-in accounts (click to switch, reloads) plus an
'Add another account' action (-> /auth/login, which uses prompt=select_account).
Trilingual. Third step of Epic N.

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

@@ -18,7 +18,9 @@
     "stats": "Statistik",
     "settings": "Einstellungen",
     "about": "Über",
-    "signOut": "Abmelden"
+    "signOut": "Abmelden",
+    "addAccount": "Weiteres Konto hinzufügen",
+    "switchTo": "Zu {{name}} wechseln"
   },
   "sync": {
     "yours": "deine",