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.

tests/unit/test_admin.py

@@ -81,6 +81,29 @@ async def test_admin_reads_and_updates_config(client):
     assert resp.json()["fee_address"] == "plm1qfeeaddress"
 
 
+async def test_admin_can_pause_and_resume_the_lottery(client):
+    headers = {"X-Admin-Token": "test-admin-token"}
+
+    resp = await client.get("/admin/config", headers=headers)
+    assert resp.json()["paused"] is False
+
+    resp = await client.post("/admin/pause", headers=headers)
+    assert resp.status_code == 200
+    assert resp.json()["paused"] is True
+
+    resp = await client.get("/admin/config", headers=headers)
+    assert resp.json()["paused"] is True
+
+    resp = await client.post("/admin/resume", headers=headers)
+    assert resp.status_code == 200
+    assert resp.json()["paused"] is False
+
+
+async def test_admin_pause_requires_token(client):
+    resp = await client.post("/admin/pause")
+    assert resp.status_code == 403
+
+
 async def test_admin_lists_users(client):
     from app.db import base as db_base
     from app.db.models import User