feat: show weekly downloads on the extension detail page

[netomi]

Stack: 6 of 6. Split out of #2027. Base: #2134 (split/download-analytics).

All three commits are @gnugomez's, cherry-picked with -x, and they applied cleanly.

The visible end of the feature: a weekly-downloads card on the extension detail page, fed by the series endpoint from #2134.

• weekly-downloads.tsx and use-extension-download-series.ts
• the series call and its types in extension-registry-service.ts / extension-registry-types.ts
• placed by extension-detail-overview.tsx

It reads analyticsEnabled from /api/version, so on a registry with analytics off nothing is requested and nothing is rendered — which is every deployment by default.

Why it is last

It is the only part of #2027 with a dependency in both directions: it needs the page primitives and Pill from #2130, and the series endpoint from #2134. Landing it earlier would have meant either stubbing the endpoint or shipping a card with nothing behind it.

Verification

yarn test 281 passing across 55 files (274 from #2130 plus 7 here), yarn lint clean, tsc --noEmit clean.

Stack summary

PR	Unit	Base
#2130	webui library surface	main
#2131	ingestion move (pure relocation)	#2130
#2132	ingestion rework	#2131
#2133	time-series database	#2132
#2134	analytics core and API	#2133
#2135	this	#2134

Two of those carry things worth deciding rather than just reviewing: #2132 is a rewrite of live download counting that #2027's "disabled by default" framing hides, and #2134 is where a synchronous time-series write lands on the download request path.

Refs #2027, #2025