feat: Info tab enriched — retroactive AI estimate, annual totals, inventory & activity stats

api/index.php:
- gemini_usage: retroactive AI call estimate from cache files (price/shelf/category)
  with per-entry token estimates (price ~475tok, shelf ~580tok, category ~230tok)
- yearly totals: sum tracked months + retro estimate for full 2026 view
- DB activity stats: products, inventory, transactions, expired, expiring_soon
- cache stats: price (255), shelf (30), category (7), foodfacts (10)
- system info: last backup timestamp+size, Bring! token expiry
- new constants: SHELF_CACHE_PATH, FOODFACTS_CACHE_PATH, BRING_TOKEN_PATH

assets/js/app.js:
- _renderInfoTab(): full rewrite — 4 cards (AI, Inventory, Activity, System)
- month displayed as localized name via Intl.DateTimeFormat (es. 'maggio 2026')
- tracked section shown when calls > 0; retro estimate always shown if gap exists
- year section: tracked + retro combined total
- pill() helper for consistent stat display

index.html: 4 cards with ids info-ai-content, info-inv-content, info-act-content, info-system-content

translations: updated settings.info.* keys in it/en/de (overview subtitle, retro labels, inv/act/system keys)

translations/en.json

@@ -771,14 +771,37 @@
       "est_cost": "Est. cost",
       "input_tok": "Input tokens",
       "output_tok": "Output tokens",
-      "ai_calls": "AI calls",
+      "ai_calls": "Calls",
       "by_action": "Breakdown by function",
       "by_model": "Breakdown by model",
-      "pricing_note": "Reference pricing: gemini-2.5-flash $0.15/1M input, $0.60/1M output.",
+      "pricing_note": "Gemini reference pricing: 2.5-flash $0.15/M in · $0.60/M out — 2.0-flash $0.10/M in · $0.40/M out.",
       "system_title": "System",
-      "db_size": "Database size",
-      "log_size": "Log size",
-      "log_level": "Log level"
+      "db_size": "Database",
+      "log_size": "Logs",
+      "log_level": "Log level",
+      "ai_overview": "AI usage overview, inventory and system status",
+      "tracked_section": "Tracked (since monitoring start)",
+      "retro_section": "Retroactive estimate — {month}",
+      "retro_note": "Estimate based on AI cache files generated before monitoring was activated.",
+      "year_section": "Year {year} — total estimate",
+      "calls_unit": "calls",
+      "inv_title": "Inventory",
+      "inv_active": "Active",
+      "inv_products": "Total products",
+      "inv_expiring": "Expiring (7d)",
+      "inv_expired": "Expired",
+      "inv_finished": "Finished",
+      "act_title": "Monthly activity",
+      "act_tx_month": "Movements",
+      "act_restock": "Restocks",
+      "act_use": "Usages",
+      "act_new_products": "New products",
+      "act_tx_year": "Yearly movements",
+      "price_cache": "Price cache",
+      "cache_entries": "products",
+      "last_backup": "Last backup",
+      "bring_days": "token expires in {n} days",
+      "bring_expired": "token expired"
     }
   },
   "expiry": {