fix: improve SPC dashboard UX — hover toolbar, visible report errors

- Change Plotly modebar to hover-only mode to avoid overlapping chart content
- Remove redundant Plotly titles (container headers already provide them)
- Add Content-Type check and inline error display for report downloads
- Fix setup login validation and seed idempotency for existing data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/templates/statistics/dashboard.html

@@ -108,6 +108,12 @@
         {{ _("Applica filtri") }}
       </button>
     </div>
+    <!-- Inline report error (shown below buttons row) -->
+    <div x-show="reportError" x-cloak
+         class="mt-2 text-sm text-red-600 dark:text-red-400 flex items-center gap-1"
+         x-text="reportError"
+         x-transition>
+    </div>
   </div>
 
   <!-- Nessuna ricetta selezionata -->
@@ -266,6 +272,7 @@ function spcDashboard() {
     loading: false,
     hasData: false,
     errorMessage: '',
+    reportError: '',
     downloadingReport: false,
 
     init() {
@@ -387,7 +394,7 @@ function spcDashboard() {
         renderHistogram('histogram-chart', this.histogram, tol);
       } else {
         const el = document.getElementById('histogram-chart');
-        if (el) el.innerHTML = '<p class="text-center text-steel py-12">{{ _("Seleziona un punto di misura per l\'istogramma") }}</p>';
+        if (el) el.innerHTML = '<p class="text-center text-steel py-12">' + {{ _("Seleziona un punto di misura per l'istogramma")|tojson }} + '</p>';
       }
 
       // Capability gauge
@@ -404,6 +411,7 @@ function spcDashboard() {
     },
 
     async downloadReport(type) {
+      this.reportError = '';
       this.downloadingReport = type;
       try {
         const params = new URLSearchParams();
@@ -417,7 +425,19 @@ function spcDashboard() {
 
         if (!resp.ok) {
           const errData = await resp.json().catch(() => ({}));
-          this.errorMessage = errData.detail || '{{ _("Errore nella generazione del report") }}';
+          const msg = errData.detail || '{{ _("Errore nella generazione del report") }}';
+          console.error('Report download failed (' + resp.status + '):', msg, errData);
+          this.reportError = msg;
+          return;
+        }
+
+        // Check Content-Type: server may return JSON error with 200 status
+        const contentType = resp.headers.get('Content-Type') || '';
+        if (!contentType.includes('application/pdf')) {
+          const errData = await resp.json().catch(() => ({}));
+          const msg = errData.detail || '{{ _("Errore nella generazione del report") }}';
+          console.error('Report returned non-PDF content-type "' + contentType + '":', msg, errData);
+          this.reportError = msg;
           return;
         }
 
@@ -434,7 +454,7 @@ function spcDashboard() {
         URL.revokeObjectURL(a.href);
       } catch (e) {
         console.error('Error downloading report:', e);
-        this.errorMessage = '{{ _("Errore di connessione al server") }}';
+        this.reportError = '{{ _("Errore di connessione al server") }}';
       } finally {
         this.downloadingReport = false;
       }