feat(pagespeed): add PageSpeed page with gauges, CWV cards, chart, and diagnostics

- ScoreGauge SVG component with color-coded circular arcs
- Full page: disabled state, score overview, CWV metrics, trend chart
- Diagnostics accordion with opportunities/diagnostics/passed groups
- Mobile/desktop strategy toggle, manual check trigger
- Loading skeleton, frequency selector

components/pagespeed/ScoreGauge.tsx

@@ -0,0 +1,71 @@
+'use client'
+
+interface ScoreGaugeProps {
+  score: number | null
+  label: string
+}
+
+const RADIUS = 44
+const CIRCUMFERENCE = 2 * Math.PI * RADIUS
+
+function getColor(score: number): string {
+  if (score >= 90) return '#0cce6b'
+  if (score >= 50) return '#ffa400'
+  return '#ff4e42'
+}
+
+export default function ScoreGauge({ score, label }: ScoreGaugeProps) {
+  const hasScore = score !== null && score !== undefined
+  const displayScore = hasScore ? Math.round(score) : null
+  const offset = hasScore ? CIRCUMFERENCE * (1 - score / 100) : CIRCUMFERENCE
+  const color = hasScore ? getColor(score) : '#6b7280'
+
+  return (
+    <div className="flex flex-col items-center gap-2">
+      <div className="relative w-[120px] h-[120px]">
+        <svg
+          className="w-full h-full -rotate-90"
+          viewBox="0 0 100 100"
+        >
+          {/* Track */}
+          <circle
+            cx="50"
+            cy="50"
+            r={RADIUS}
+            fill="none"
+            stroke="currentColor"
+            className="text-neutral-200 dark:text-neutral-700"
+            strokeWidth="8"
+          />
+          {/* Filled arc */}
+          <circle
+            cx="50"
+            cy="50"
+            r={RADIUS}
+            fill="none"
+            stroke={color}
+            strokeWidth="8"
+            strokeLinecap="round"
+            strokeDasharray={CIRCUMFERENCE}
+            strokeDashoffset={offset}
+            style={{ transition: 'stroke-dashoffset 0.6s ease' }}
+          />
+        </svg>
+        {/* Score text */}
+        <div className="absolute inset-0 flex items-center justify-center">
+          <span
+            className="text-2xl font-bold"
+            style={{ color: hasScore ? color : undefined }}
+          >
+            {displayScore !== null ? displayScore : (
+              <span className="text-neutral-400 dark:text-neutral-500">--</span>
+            )}
+          </span>
+        </div>
+      </div>
+      <span className="text-sm font-medium text-neutral-600 dark:text-neutral-400">
+        {label}
+      </span>
+    </div>
+  )
+}