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)'

data/ai_usage.json

@@ -0,0 +1,9 @@
+{
+  "2026-05": {
+    "input_tokens": 4438300,
+    "output_tokens": 1286760,
+    "calls": 8374,
+    "by_action": {},
+    "by_model": {}
+  }
+}