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/api/routes/rounds.py

@@ -25,6 +25,7 @@ class CurrentRoundResponse(BaseModel):
     winner_user_id: int | None = None
     winner_amount_sats: int | None = None
     chain_tip_height: int | None = None
+    lottery_paused: bool = False
 
 
 @router.get("/current", response_model=CurrentRoundResponse)
@@ -39,6 +40,7 @@ async def current_round(request: Request, session: AsyncSession = Depends(get_se
             bet_amount_sats=config.bet_amount_sats,
             draw_animation_seconds=config.draw_animation_seconds,
             chain_tip_height=chain_tip_height,
+            lottery_paused=config.paused,
         )
 
     participant_count = await session.scalar(
@@ -60,4 +62,5 @@ async def current_round(request: Request, session: AsyncSession = Depends(get_se
         winner_user_id=round_.winner_user_id,
         winner_amount_sats=round_.winner_amount_sats,
         chain_tip_height=chain_tip_height,
+        lottery_paused=config.paused,
     )