Add Umami tracking, matching the data explorer #97

Add Umami to the BlockExplorer frontend the way the data explorer does it: the umami.arkiv.network script in frontend/index.html, a track() helper for custom events, and an events table in the README.

The BlockExplorer reports no usage numbers, so there is no way to tell which pages and filters get used once the restyle lands. The explorer's setup (app/layout.tsx, lib/analytics.ts, README Analytics section) is the reference: no PII, fixed labels or buckets only.

How the data explorer does it

• app/layout.tsx loads https://umami.arkiv.network/script.js deferred, with data-website-id and data-exclude-search="true".
• lib/analytics.ts exports track(name, data). It is a no-op when the script is blocked, so callers never guard.
• Plain links use data-umami-event and data-umami-event-url attributes instead of a handler.
• The README has an Analytics section with one table row per event. Adding or renaming an event updates the table in the same PR.
• No PII. Every event value is a fixed label or a bucket. No query text, keys, or addresses.

Proposed setup for the block explorer

1. Own website in Umami. Create a second website for the block explorer so its numbers do not mix with the data explorer's.

2. Website id from the environment. Add VITE_UMAMI_WEBSITE_ID to RUNTIME_CONFIG_ENV_NAMES in frontend/server.js so it ships through /config.js like the other settings. In frontend/index.html, after the config.js script, a small inline snippet appends the Umami script only when the id is set. Local dev and forks then send nothing.

3. Manual page views. The block explorer routes with ?view= and puts block numbers, addresses, and entity keys in the query string. The explorer's data-exclude-search would fold every page into /, and sending the query would leak addresses. Set data-auto-track="false" and send a page view on every view change with the view name as the url, for example /blocks or /entity, and nothing else.

4. Helper. Copy lib/analytics.ts to frontend/src/analytics.ts unchanged.

5. First event set. Mirror the explorer's names where the meaning is the same.

   Event	Data	Fires when
   query-executed	source (node, index, compare), queryType (key-lookup, owner, expression), trigger (button, keyboard, example, shared-link)	a Data page query is submitted
   query-result	outcome (results, empty, error), resultBucket (0, 1-10, 11-50, 51+)	the query returns or fails
   entity-action	action (filter, add-to-query, copy-value, copy-link, copy-payload)	an action is taken on an entity card
   filter-applied	view, field	a list filter is submitted on Blocks, Address, Senders, Records, or Ranges
   copy	kind (hash, address, block, link)	a copy button is used
   chart-selected	chart	a chart is picked on the Charts page
   theme-toggled	theme (light, dark)	the header toggle is used
   outbound-link-click	url	Feedback, llms.txt, or docs links are followed

6. README. Add an Analytics section with the table above and the same no-PII note as the explorer.