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/components/Comparison.tsx

@@ -197,7 +197,7 @@ function InlinePicker({ start, end, onChange, onClose, availableYears, L }: {
   };
 
   return (
-    <div className="alt-picker">
+    <div className="alt-picker" id="period-picker-panel">
       <div className="alt-picker-year">
         <button type="button" onClick={() => shift(L.dir==='rtl' ? 1 : -1)} disabled={L.dir==='rtl' ? year>=maxY : year<=minY} className="alt-yr-btn">
           <svg width="7" height="11" viewBox="0 0 7 11" fill="none"><path d="M5.5 9.5L1.5 5.5L5.5 1.5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round"/></svg>
@@ -262,7 +262,7 @@ function PeriodCard({ role, hint, start, end, variant, onChange, availableYears,
         </div>
         <div className="alt-period-name">{periodNameL(start, end, L)}</div>
         <div className="alt-date-range">{dateRangeTextL(start, end, L)}</div>
-        <button type="button" className="alt-change-btn" onClick={() => setOpen(v => !v)} aria-expanded={open}>
+        <button type="button" className="alt-change-btn" onClick={() => setOpen(v => !v)} aria-expanded={open} aria-controls="period-picker-panel">
           {open ? L.close : L.changePeriod}
           <svg width="10" height="10" viewBox="0 0 10 10" fill="none" style={{ transform:open?'rotate(180deg)':'none', transition:'transform 0.2s' }}>
             <path d="M2 3.5L5 6.5L8 3.5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round"/>
@@ -443,7 +443,10 @@ export default function PeriodSelectorDemo({ data, seasons, includeVAT, allowedM
   }, [data, prevData, currData, prevStart, prevEnd, currStart, currEnd, metric, getVal]);
 
   const baseOpts = useMemo(() => createBaseOptions(false), []);
-  const chartOpts: any = { ...baseOpts, plugins:{ ...baseOpts.plugins, legend:{ position:'top', align:'end', labels:{ boxWidth:12, padding:12 } } } };
+  const { chartOpts } = useMemo(() => {
+    const chartOpts: any = { ...baseOpts, plugins:{ ...baseOpts.plugins, legend:{ position:'top', align:'end', labels:{ boxWidth:12, padding:12 } } } };
+    return { chartOpts };
+  }, [baseOpts]);
 
   const metricOpts = [
     { value:'revenue', label:L.revenue }, { value:'visitors', label:L.visitors },
@@ -477,7 +480,7 @@ export default function PeriodSelectorDemo({ data, seasons, includeVAT, allowedM
       <style>{`
         ${L.fontImport}
 
-        .alt-page { max-width:1100px; margin:0 auto; padding:48px 24px 80px; font-family:${L.bodyFont}; width:100%; box-sizing:border-box; }
+        .alt-page { max-width:1400px; margin:0 auto; padding:48px 24px 80px; font-family:${L.bodyFont}; width:100%; box-sizing:border-box; }
 
         /* ── header ── */
         .alt-back { display:inline-flex; align-items:center; gap:6px; font-size:.8125rem; color:var(--text-muted); text-decoration:none; margin-bottom:28px; transition:color .15s; }
@@ -572,7 +575,7 @@ export default function PeriodSelectorDemo({ data, seasons, includeVAT, allowedM
         .alt-filter-reset:hover { color:var(--danger); }
 
         /* ── metrics ── */
-        .alt-metrics { display:grid; grid-template-columns:repeat(3,1fr); gap:1px; background:var(--border); border:1px solid var(--border); border-radius:var(--radius); overflow:hidden; margin-bottom:40px; }
+        .alt-metrics { display:grid; grid-template-columns:repeat(auto-fit,minmax(200px,1fr)); gap:1px; background:var(--border); border:1px solid var(--border); border-radius:var(--radius); overflow:hidden; margin-bottom:40px; }
         .alt-metric { background:var(--surface); padding:24px 22px; }
         .alt-metric-title { font-size:.6875rem; font-weight:700; text-transform:uppercase; letter-spacing:.08em; color:var(--text-muted); margin:0 0 12px; }
         .alt-metric-value { font-family:${L.displayFont}; font-size:1.875rem; font-weight:400; color:var(--text-primary); line-height:1; margin-bottom:10px; letter-spacing:-.02em; }
@@ -612,6 +615,13 @@ export default function PeriodSelectorDemo({ data, seasons, includeVAT, allowedM
           .alt-chart-header { flex-direction:column; }
         }
 
+        @media (max-width:700px) {
+          .alt-filter-bar { overflow-x:auto; flex-wrap:nowrap; padding-inline:12px; -webkit-overflow-scrolling:touch; }
+          .altms-trigger { flex-shrink:0; }
+          .alt-vat-toggle { flex-shrink:0; }
+          .alt-filter-spacer { display:none; }
+        }
+
         /* ── focus-visible ── */
         .alt-chip:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
         .alt-ctrl:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }