feat: add PostHog analytics integration

[posthog-eu]

Summary

This PR adds PostHog product analytics to the web viewer (packages/web, a Vite app) using the posthog-js SDK.

• packages/web/src/posthog.ts — new PostHog client initialization, reading the token/host from VITE_PUBLIC_POSTHOG_KEY / VITE_PUBLIC_POSTHOG_HOST, with browser exception autocapture enabled (capture_unhandled_errors, capture_unhandled_rejections).
• packages/web/src/main.ts — captures command_copied when a copy-command button is used.
• packages/web/src/viewer/app.js — captures note_opened (with open_mode), note_closed, and search_result_opened at the documentation viewer's interaction handlers.
• packages/web/package.json, pnpm-lock.yaml — adds the posthog-js dependency.
• packages/cli/cmd/openknowledge/viewer_assets/viewer.js — regenerated bundled viewer asset reflecting the above (produced by the existing build:viewer script).
• pnpm-workspace.yaml — minor pnpm build-approval bookkeeping picked up while installing the new dependency.

The app is unauthenticated, so no persistent user identification was added — only anonymous, personless event capture.

Insights and dashboards created

Type	Name	Link
Dashboard	Your starter dashboard	https://eu.posthog.com/project/238777/dashboard/867438
Dashboard	Analytics basics (wizard)	https://eu.posthog.com/project/238777/dashboard/867457
Insight	Active users (last 30 days)	https://eu.posthog.com/project/238777/insights/fmmlg8BP
Insight	Sessions (last 7 days)	https://eu.posthog.com/project/238777/insights/yyoo8VJT
Insight	Pageviews (last 7 days)	https://eu.posthog.com/project/238777/insights/FZbm0VVs
Insight	Daily active users (DAUs)	https://eu.posthog.com/project/238777/insights/z09IBCE5
Insight	Weekly active users (WAUs)	https://eu.posthog.com/project/238777/insights/SPelKZq3
Insight	Retention	https://eu.posthog.com/project/238777/insights/mSZlzzni
Insight	Top referrers	https://eu.posthog.com/project/238777/insights/icFgMOUk
Insight	Visit to interaction funnel	https://eu.posthog.com/project/238777/insights/dC58VYO6
Insight	Documentation engagement events (wizard)	https://eu.posthog.com/project/238777/insights/X313dhOU
Insight	Note opens by mode (wizard)	https://eu.posthog.com/project/238777/insights/ENXUTg3C
Insight	Search-to-note journey (wizard)	https://eu.posthog.com/project/238777/insights/tI1VnBmO

How to verify

1. Deploy this branch (or run pnpm dev / pnpm build && pnpm start locally — the env vars from the section below are already committed).
2. Open the docs viewer and click around: copy a command snippet, open a note, use search and click a result, close a note panel.
3. In PostHog, open Activity — you should see command_copied, note_opened, note_closed, and search_result_opened events arriving within a minute.
4. Check the Your starter dashboard and Analytics basics (wizard) dashboards for data.

Environment variables

Production is deployed to Railway as a Docker image (Dockerfile + railway.json), and the build runs pnpm build:web (which invokes vite build) directly inside the Docker build stage via COPY . . — Railway dashboard variables are not passed into that build step. Because Vite bakes VITE_* variables in at build time, the required variables were committed directly to packages/web/.env (also mirrored in .env.example) in a separate commit on this branch, so PostHog will start working automatically as soon as this branch is deployed to production. No manual action is needed.

• VITE_PUBLIC_POSTHOG_KEY — the project API token
• VITE_PUBLIC_POSTHOG_HOST — https://eu.i.posthog.com

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Created with PostHog Code

[posthog-eu]

CI status

Three checks are red on this PR, but all three already fail on main at the base commit (3152cea) and are unrelated to the PostHog integration added here:

• Dependency vulnerabilities — fails on a pre-existing critical CVE in the vitest@3.2.4 devDependency (GHSA-5xrq-8626-4rwp, fixed in 3.2.6). vitest was not touched by this PR.
• Go tests (macos-latest) — fails on TestRunConnectAndRefreshPreserveGitRefAndSubdir, a CLI git-ref/subdir test unrelated to any file changed here.
• Go tests (windows-latest) — same pre-existing failure pattern as macOS, unrelated to this PR's changes.

verify, Go tests (ubuntu-latest), Node 18 compatibility, and all CodeQL checks pass.

Since these three failures predate this branch and aren't caused by the PostHog changes, I've left them as-is per this task's scope (integration + prod env vars + minimal CI fixes only).