Fix week overlay alignment, in-plot ISO week legend, shorter charts

- weeklySeries shifts overlaid weeks onto the current week's time axis so
  they overlay inside the plot instead of overflowing left; oldest weeks
  paint first, current week on top
- Legend moved inside the visits chart's top right: current ISO week in
  accent, past weeks as a single muted "Week M" / "Week M–N" specimen
- Past week curves use the muted color instead of faded accent
- Chart height reduced ~30% (180 -> 126)

frontend/src/VisitorCharts.vue

@@ -37,6 +37,12 @@ function freqLabel(unit) {
   return unit === '5min' ? '5 min' : unit === 'hour' ? 'hourly' : 'daily'
 }
 
+/** Legend label for the overlaid past weeks: "Week M" or "Week M–N". */
+function pastLabel(series) {
+  const oldest = series.at(-1).label.slice(5) // strip "Week "
+  return series.length > 2 ? `Week ${oldest}–${series[1].label.slice(5)}` : `Week ${oldest}`
+}
+
 const now = ref(Date.now())
 let refreshInterval = null
 onMounted(() => {
@@ -52,8 +58,8 @@ const viewChart = computed(() => buildChart(viewSeries.value, now.value))
 
 <template>
   <section v-for="c in [
-      { ylabel: 'visits', chart: visitChart, empty: 'no visits recorded yet' },
-      { ylabel: 'views', chart: viewChart, empty: 'no views recorded yet' },
+      { ylabel: 'visits', chart: visitChart, legend: true, empty: 'no visits recorded yet' },
+      { ylabel: 'views', chart: viewChart, legend: false, empty: 'no views recorded yet' },
     ]" :key="c.ylabel">
     <template v-if="c.chart">
       <svg class="chart" :viewBox="`${-MARGIN_L} 0 ${VIEW_W} ${VIEW_H}`"
@@ -70,9 +76,11 @@ const viewChart = computed(() => buildChart(viewSeries.value, now.value))
           <path :d="c.chart.skyline" class="line" />
         </template>
         <template v-else>
-          <template v-for="(s, i) in c.chart.series" :key="i">
+          <!-- Oldest overlay weeks first so the current week paints on top. -->
+          <template v-for="(s, i) in [...c.chart.series].reverse()" :key="i">
             <path v-if="s.area" :d="s.area" class="area" />
-            <path :d="s.line" class="line" :style="{ opacity: s.opacity }" />
+            <path :d="s.line" class="line" :class="{ past: s.past }"
+                  :style="{ opacity: s.opacity }" />
           </template>
         </template>
         <line :x1="0" :x2="CHART_W" :y1="CHART_H - 0.5" :y2="CHART_H - 0.5"
@@ -84,12 +92,18 @@ const viewChart = computed(() => buildChart(viewSeries.value, now.value))
               class="yaxis-label">{{ freqLabel(c.chart.unit) }} {{ c.ylabel }}</text>
         <text v-for="t in c.chart.xticks" :key="'x' + t.x" :x="t.x" :y="CHART_H + MARGIN_B - 8"
               text-anchor="middle" class="xlab">{{ t.label }}</text>
+        <!-- Week overlay legend, top right inside the plot: current week in
+             accent, one muted specimen for the whole past range. -->
+        <g v-if="c.legend && c.chart.series.length > 1">
+          <line :x1="CHART_W - 86" :x2="CHART_W - 66" y1="10" y2="10" class="line" />
+          <text :x="CHART_W - 60" y="10" dominant-baseline="middle"
+                class="leglab">{{ c.chart.series[0].label }}</text>
+          <line :x1="CHART_W - 86" :x2="CHART_W - 66" y1="23" y2="23"
+                class="line past" style="opacity: 0.6" />
+          <text :x="CHART_W - 60" y="23" dominant-baseline="middle"
+                class="leglab">{{ pastLabel(c.chart.series) }}</text>
+        </g>
       </svg>
-      <div v-if="c.chart.series && c.chart.series.length > 1" class="legend">
-        <span v-for="(s, i) in c.chart.series" :key="i" :style="{ opacity: s.opacity }">
-          ● {{ s.label }}
-        </span>
-      </div>
     </template>
     <p v-else class="empty">{{ c.empty }}</p>
   </section>
@@ -115,6 +129,11 @@ const viewChart = computed(() => buildChart(viewSeries.value, now.value))
   font-variant-numeric: tabular-nums;
 }
 
+.chart .leglab {
+  font-size: 10px;
+  fill: var(--muted);
+}
+
 .chart .minor {
   stroke: var(--line);
   stroke-width: 1;
@@ -159,16 +178,11 @@ const viewChart = computed(() => buildChart(viewSeries.value, now.value))
   stroke-linecap: round;
 }
 
-.legend {
-  display: flex;
-  gap: 1.2rem;
-  margin-top: 0.4rem;
-  font-size: 0.75rem;
-  color: var(--muted);
+/* Past overlay weeks contrast with the current week's accent color. */
+.chart .line.past {
+  stroke: var(--muted);
 }
 
-.legend span { color: var(--accent); }
-
 section { margin-top: 1.8rem; }
 
 .empty { color: var(--muted); }

frontend/src/analytics/chart.js

@@ -10,7 +10,7 @@ import { DAY, HOUR, MIN5, WEEK, mondayUTC } from './time.js'
 import { formatCount } from './format.js'
 
 export const CHART_W = 720
-export const CHART_H = 180
+export const CHART_H = 126
 export const PAD_TOP = 14 // room above the highest point
 export const MARGIN_L = 56 // y tick labels + vertical axis label
 export const MARGIN_B = 24 // x tick labels

frontend/src/analytics/time.js

@@ -26,6 +26,15 @@ export function mondayUTC(t) {
   return (d - ((d + 3) % 7)) * DAY
 }
 
+/** ISO 8601 week number of the week containing t (via its Thursday). */
+export function isoWeek(t) {
+  const d = new Date(t)
+  d.setUTCHours(0, 0, 0, 0)
+  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7))
+  const yearStart = Date.UTC(d.getUTCFullYear(), 0, 1)
+  return Math.ceil(((d - yearStart) / DAY + 1) / 7)
+}
+
 /** Parse sparse timestamp buckets into a { epochMs: count } map. */
 export function rawTimes(buckets) {
   const raw = {}
@@ -44,7 +53,9 @@ export function sumRange(raw, t0, t1) {
 /**
  * One series per overlaid week: [this week, 1 week ago, ...], at native
  * 5-minute resolution, up to 8 weeks back (and only weeks that overlap the
- * recorded data at all). The current week is truncated at the current bucket
+ * recorded data at all). Each older week's timestamps are shifted forward
+ * onto the current week's axis so all curves overlay inside the plot.
+ * The current week is truncated at the current bucket
  * — no fake zeroes drawn for the future. Counts are rates per hour
  * (bucket count * 12): a lone visit in a 5-minute bucket reads as "12/h".
  * The coarser ranges use per-day rates instead (unitMinutes = 24*60).
@@ -67,12 +78,13 @@ export function weeklySeries(buckets) {
       : start + WEEK
     const points = []
     for (let t = start; t < end; t += MIN5) {
-      points.push({ t, count: raw[t] || 0 })
+      points.push({ t: t + back * WEEK, count: raw[t] || 0 })
     }
     out.push({
       points,
-      label: back === 0 ? 'this week' : `${back}w ago`,
+      label: `Week ${isoWeek(start)}`,
       opacity: Math.max(0.15, 1 - back * 0.25),
+      past: back > 0,
       area: back === 0,
     })
   }