Add a maintenance pause/resume switch and a proper user navbar

RoundConfig gets a paused flag toggled via new POST /admin/pause and
/admin/resume endpoints (audit-logged, surfaced as a "Manutenzione" card in
the admin Parametri view). Pausing only stops the *next* round from opening
once the current one closes — rounds/service.py:open_new_round_if_needed
still lets an in-progress round finish, draw, and pay out its winner
normally. GET /rounds/current exposes lottery_paused so the user page shows
a maintenance banner (even while logged out) instead of silently going idle.

Also replaces the user dashboard's stacked account-bar card + bento-grid
menu with a single sticky navbar (identity row + Deposito/Bet/Prelievo
tabs), and moves the page content into a dedicated .app-shell container so
the navbar itself can span full width.

app/db/models.py

@@ -95,6 +95,10 @@ class RoundConfig(Base):
     min_amount_sats: Mapped[int] = mapped_column(BigInteger, default=100_000_000)
     fee_rate_sat_vb: Mapped[int] = mapped_column(default=1)
     rbf_timeout_seconds: Mapped[int] = mapped_column(default=900)
+    # Maintenance switch: when true, the round currently in progress still runs to
+    # completion (closes, draws, pays out the winner) but no new round is opened
+    # afterwards — see rounds/service.py:open_new_round_if_needed.
+    paused: Mapped[bool] = mapped_column(default=False)
     updated_at: Mapped[datetime] = mapped_column(default=utcnow, onupdate=utcnow)