add posthog analytics for llm usage and auth events

Captures per-LLM-call token usage tagged by feature (copilot chat,
track block, meeting note, knowledge sync), plus sign-in / sign-out
and identity. Renderer and main share one PostHog identity so events
from either process resolve to the same user.

See apps/x/ANALYTICS.md for the event catalog, person properties,
use-case taxonomy, and how to add new events.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/x/pnpm-lock.yaml

@@ -404,6 +404,9 @@ importers:
       pdf-parse:
         specifier: ^2.4.5
         version: 2.4.5
+      posthog-node:
+        specifier: ^4.18.0
+        version: 4.18.0
       react:
         specifier: ^19.2.3
         version: 19.2.3
@@ -6471,6 +6474,10 @@ packages:
   posthog-js@1.332.0:
     resolution: {integrity: sha512-w3+sL+IFK4mpfFmgTW7On8cR+z34pre+SOewx+eHZQSYF9RYqXsLIhrxagWbQKkowPd4tCwUHrkS1+VHsjnPqA==}
 
+  posthog-node@4.18.0:
+    resolution: {integrity: sha512-XROs1h+DNatgKh/AlIlCtDxWzwrKdYDb2mOs58n4yN8BkGN9ewqeQwG5ApS4/IzwCb7HPttUkOVulkYatd2PIw==}
+    engines: {node: '>=15.0.0'}
+
   postject@1.0.0-alpha.6:
     resolution: {integrity: sha512-b9Eb8h2eVqNE8edvKdwqkrY6O7kAwmI8kcnBv1NScolYJbo59XUF0noFq+lxbC1yN20bmC0WBEbDC5H/7ASb0A==}
     engines: {node: '>=14.0.0'}
@@ -15203,6 +15210,12 @@ snapshots:
       query-selector-shadow-dom: 1.0.1
       web-vitals: 4.2.4
 
+  posthog-node@4.18.0:
+    dependencies:
+      axios: 1.13.2
+    transitivePeerDependencies:
+      - debug
+
   postject@1.0.0-alpha.6:
     dependencies:
       commander: 9.5.0