feat: Generali tab, time-based auto theme, AI cost from real data

- index.html: new Generali tab (first, active) with Language/Currency/
  Theme/Screensaver/ZeroWaste/Export; old tab-language removed;
  screensaver timeout select uses form-input style; asset v=20260518a
- app.js: auto theme = time-based (20:00-07:00 dark, not system pref);
  removed matchMedia listener; added 5min setInterval for auto re-check;
  removed Bring! token row from Info tab (internal implementation detail)
- api/index.php: gemini_usage - removed all cache-estimation code;
  month/year_stats from ai_usage.json only
- data/ai_usage.json: data-driven baseline estimate for 2026-05:
  ~4.4M in + ~1.3M out from 8374 inferred historical calls (102 recipes,
  555 price lookups, getStats loop pre-fix, smart cron runs, etc.)
  = ~EUR 1.32 at 2.5-flash rates; new calls tracked precisely from now
- translations: settings.tab_general added; theme.auto updated to
  'Automatico (orario)' / 'Automatic (time of day)' / 'Automatisch (Tageszeit)'

translations/it.json

@@ -755,7 +755,7 @@
       "label": "🌙 Tema",
       "off": "☀️ Chiaro",
       "on": "🌙 Scuro",
-      "auto": "🔄 Automatico (sistema)"
+      "auto": "🔄 Automatico (orario)"
     },
     "zerowaste": {
       "card_title": "♻️ Suggerimenti zero-waste",
@@ -801,7 +801,8 @@
       "year_label": "Anno {year}",
       "currency_title": "Valuta",
       "currency_hint": "La valuta usata per tutti i costi e i prezzi nell'app."
-    }
+    },
+    "tab_general": "Generali"
   },
   "expiry": {
     "today": "OGGI",