feat: timer intervallo misura su ricetta + auto-logout per inattività

- Recipe: nuovo campo measurement_interval_minutes (migration 003) con UI nel recipe editor
- Auto-logout: impostazione di sistema configurabile da admin/settings, timer inattività JS
  con warning modale 60s prima del logout, tracking eventi utente throttled
- Navbar: aggiunto link "Impostazioni" per admin (desktop + mobile)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/frontend/flask_app/templates/base.html

@@ -115,6 +115,74 @@
 
   {% block extra_js %}{% endblock %}
 
+  {% if current_user and auto_logout_minutes %}
+  <script>
+  function inactivityTimer() {
+    var timeoutMs = {{ auto_logout_minutes }} * 60 * 1000;
+    var warningMs = 60 * 1000;
+    return {
+      lastActivity: Date.now(),
+      showWarning: false,
+      remainingSeconds: 60,
+      _interval: null,
+      init() {
+        var self = this;
+        this._interval = setInterval(function() { self._check(); }, 5000);
+        var lastUpdate = 0;
+        var handler = function() {
+          var now = Date.now();
+          if (now - lastUpdate > 3000) {
+            lastUpdate = now;
+            self.lastActivity = now;
+            self.showWarning = false;
+          }
+        };
+        ['mousemove','mousedown','keypress','touchstart','scroll'].forEach(function(evt) {
+          window.addEventListener(evt, handler, { passive: true });
+        });
+      },
+      resetTimer() {
+        this.lastActivity = Date.now();
+        this.showWarning = false;
+      },
+      _check() {
+        var elapsed = Date.now() - this.lastActivity;
+        var remaining = timeoutMs - elapsed;
+        if (remaining <= 0) {
+          clearInterval(this._interval);
+          window.location.href = '{{ url_for("auth.logout") }}';
+        } else if (remaining <= warningMs) {
+          this.showWarning = true;
+          this.remainingSeconds = Math.ceil(remaining / 1000);
+        }
+      }
+    };
+  }
+  </script>
+  <div x-data="inactivityTimer()" class="contents">
+    <template x-if="showWarning">
+      <div class="fixed inset-0 z-[100] flex items-center justify-center bg-black/50" @click.self="resetTimer()">
+        <div class="bg-[var(--bg-primary)] rounded-xl p-6 shadow-2xl max-w-sm mx-4 border border-[var(--border-color)]">
+          <div class="flex items-center gap-3 mb-4">
+            <div class="w-10 h-10 rounded-full bg-amber-100 dark:bg-amber-900/30 flex items-center justify-center">
+              <svg class="w-5 h-5 text-amber-600" fill="none" stroke="currentColor" stroke-width="2" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" d="M12 8v4l3 3m6-3a9 9 0 11-18 0 9 9 0 0118 0z"/>
+              </svg>
+            </div>
+            <h3 class="text-lg font-semibold text-[var(--text-primary)]">{{ _('Sessione in scadenza') }}</h3>
+          </div>
+          <p class="text-sm text-[var(--text-secondary)] mb-5">
+            {{ _('Sarai disconnesso tra') }} <span class="font-mono font-bold text-amber-600" x-text="remainingSeconds"></span> {{ _('secondi per inattività.') }}
+          </p>
+          <button @click="resetTimer()" class="btn btn-primary w-full">
+            {{ _('Continua a lavorare') }}
+          </button>
+        </div>
+      </div>
+    </template>
+  </div>
+  {% endif %}
+
   <!-- Alpine.js CDN (defer) - must load AFTER extra_js so component functions are defined -->
   <script defer src="https://cdn.jsdelivr.net/npm/alpinejs@3.x.x/dist/cdn.min.js"></script>
 </body>