feat: native shopping list — decouple from Bring! (#105)

- New shopping_list SQLite table (migration in migrateDB)
- shoppingGetList/Add/Remove — delegates to Bring! or internal DB
  based on SHOPPING_MODE env var (default: internal)
- isShoppingBringMode() guard: requires mode=bring + BRING credentials
- bringQuickSyncProduct updated to support both modes
- All bring_* JS calls replaced with shopping_* (bring_migrate_names kept)
- New settings tab 'Lista spesa' (tab-bring) with:
  - Enable/disable shopping list toggle
  - Provider radio: internal vs Bring!
  - Bring! sub-section (shown only when mode=bring)
  - AI smart suggestions toggle
  - Forecast toggle
  - Auto-add threshold (qty slider)
  - Price estimation section
- _applyShoppingSettingsUI, onShoppingEnabledChange, onShoppingModeChange
- SHOPPING_* env vars documented in .env.example
- cron_smart_shopping respects SHOPPING_MODE and SHOPPING_SMART_SUGGESTIONS
- Translations: 12 new keys in all 5 languages (it/en/de/fr/es)
- DB busy_timeout=5000ms + WAL pragma in getDB() (fixes #95)

translations/en.json

@@ -849,7 +849,22 @@
       "currency_title": "Currency",
       "currency_hint": "The currency used for all costs and prices in the app."
     },
-    "tab_general": "General"
+    "tab_general": "General",
+    "shopping": {
+      "tab": "Shopping list",
+      "title": "Shopping list",
+      "hint": "Configure the built-in shopping list or connect Bring!.",
+      "enable_label": "Enable shopping list",
+      "mode_label": "Provider",
+      "mode_internal": "Built-in (no Bring!)",
+      "mode_bring": "Bring! (external app)",
+      "bring_section_title": "Bring! configuration",
+      "ai_section_title": "AI assistance",
+      "smart_suggestions_label": "AI suggestions",
+      "forecast_label": "Forecast low-stock products",
+      "auto_add_label": "Auto-add to list when",
+      "auto_add_suffix": "remaining in stock (0 = only when empty)"
+    }
   },
   "expiry": {
     "today": "TODAY",