feed: add public popularity (views-over-time) chart in the ⋯ menu (#26)

A "Popularity" item in the video ⋯ menu opens a public views-over-time chart
(view data is public). Builds on the now-trustworthy view counts (#4).

- buildViewCountTimeSeries buckets raw kind-30079 view events by the counter's
  dedupe window (one view per viewer per day) into a cumulative series.
- buildViewCountChartSvg renders a token-colored (currentColor / text-accent)
  SVG area+line chart — no charting library, no raw colors.
- The modal fetches via listVideoViewEventsWithDefaultClient, shows the total,
  and updates LIVE by subscribing to view events (debounced re-render); cleans
  up on close (Esc / backdrop / ✕). Lazy-loaded from the menu handler so it
  only ships when opened.

New size tokens for the modal; reuses .ds-overlay-backdrop for the scrim.

Tests: tests/view-count-chart.test.mjs (cumulative dedupe, junk filtering,
SVG line/area + empty/single-point).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

css/tailwind.source.css

@@ -518,6 +518,21 @@
     -webkit-backdrop-filter: blur(var(--overlay-backdrop-blur));
   }
 
+  .popularity-modal__backdrop {
+    z-index: var(--z-overlay-popover);
+    padding: var(--space-md);
+  }
+
+  .popularity-modal {
+    width: 100%;
+    max-width: var(--popularity-modal-max-width);
+  }
+
+  .popularity-modal__chart {
+    width: 100%;
+    height: var(--popularity-modal-chart-height);
+  }
+
   .popover-panel[data-variant="zap"],
   .popover__panel[data-variant="zap"] {
     background: var(--color-overlay-veil);

css/tokens.css

@@ -80,6 +80,8 @@
   --blur-modal-alert: 14px;
   --blur-thumbnail-blur: 18px;
   --modal-nav-backdrop-blur: 6px;
+  --popularity-modal-max-width: 28rem;
+  --popularity-modal-chart-height: 130px;
 
   --app-header-centerline-offset: calc(
     var(--space-sm) + (var(--space-2xl-plus) / 2)