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/static/js/spc-charts.js

@@ -35,14 +35,14 @@ const PLOTLY_LAYOUT_DEFAULTS = {
   font: { family: 'Inter, system-ui, sans-serif', color: SPC_COLORS.textColor },
   paper_bgcolor: 'rgba(0,0,0,0)',
   plot_bgcolor: 'rgba(0,0,0,0)',
-  margin: { t: 40, r: 30, b: 50, l: 60 },
+  margin: { t: 20, r: 30, b: 50, l: 60 },
   xaxis: { gridcolor: SPC_COLORS.gridColor, zeroline: false },
   yaxis: { gridcolor: SPC_COLORS.gridColor, zeroline: false },
 };
 
 const PLOTLY_CONFIG = {
   responsive: true,
-  displayModeBar: true,
+  displayModeBar: 'hover',
   modeBarButtonsToRemove: ['lasso2d', 'select2d'],
   displaylogo: false,
 };
@@ -154,7 +154,6 @@ function renderControlChart(containerId, data) {
 
   const layout = {
     ...PLOTLY_LAYOUT_DEFAULTS,
-    title: { text: _t('controlChart'), font: { size: 14 } },
     xaxis: { ...PLOTLY_LAYOUT_DEFAULTS.xaxis, title: _t('measureNum') },
     yaxis: { ...PLOTLY_LAYOUT_DEFAULTS.yaxis, title: _t('value') },
     shapes,
@@ -231,7 +230,6 @@ function renderHistogram(containerId, data, tol) {
 
   const layout = {
     ...PLOTLY_LAYOUT_DEFAULTS,
-    title: { text: _t('histogram'), font: { size: 14 } },
     xaxis: { ...PLOTLY_LAYOUT_DEFAULTS.xaxis, title: _t('value') },
     yaxis: { ...PLOTLY_LAYOUT_DEFAULTS.yaxis, title: _t('frequency') },
     shapes,