fix: responsive, ARIA, performance and CSS cleanup improvements

Addresses remaining medium and low severity audit findings:

- H2: Dark mode @media selector narrowed to :root:not([data-theme]) so
  OS-preference and manual-override blocks are now mutually exclusive
- L2: Remove ~410 lines of dead Slides Builder CSS (no component exists)
- M2: VAT toggle uses flex spacer instead of margin-inline-start:auto,
  preventing layout break when filter bar wraps at medium-small widths
- M3: Page content max-width aligned to 1400px (matches nav bar)
- M5: Period picker toggle now has aria-controls="period-picker-panel"
  pointing to the InlinePicker root in both Dashboard and Comparison
- M6: Offline badge cache timestamp exposed via sr-only span for
  screen reader accessibility (was title-only before)
- M7: chartOpts/barHorizOpts/barNoLegend wrapped in useMemo([baseOpts])
  to prevent unnecessary Chart.js re-renders
- L4: Filter bar scrolls horizontally on mobile instead of wrapping
- L5: Metrics grid uses auto-fit/minmax to eliminate orphaned cards

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

src/App.tsx

@@ -250,6 +250,11 @@ function App() {
                     <line x1="12" y1="20" x2="12.01" y2="20"/>
                   </svg>
                   {t('app.offline') || 'Offline'}
+                  {cacheInfo && (
+                    <span className="sr-only">
+                      {` (cached ${new Date(cacheInfo.timestamp || '').toLocaleString()})`}
+                    </span>
+                  )}
                 </span>
               )}
               <button