feat(analytics): add consent-gated activation-funnel analytics via PostHog

Installs are up 615% but 7-day retention is ~0% and we had no analytics SDK
to see where users drop off. Adds a thin PostHog wrapper (src/lib/analytics.ts)
that tracks app_opened, connection_form_submitted, connection_attempted,
connection_succeeded/failed (with a coarse error_class, e.g. the known 401
auth bug), message_sent, and response_received.

PostHog was chosen over Aptabase for its GMS-free JS-only RN SDK (fine for
the F-Droid/no-Firebase build), EU-hosted/self-host option, and generous
free tier. Analytics shares the exact same consent flag as Sentry
(telemetry.ts now gates both) so zero network calls happen without explicit
opt-in.

Requires a new EXPO_PUBLIC_POSTHOG_KEY CI secret (wired into build.yml,
publish-fdroid.yml, publish-play-store.yml, and documented in
publish-app-store.yml alongside the existing Sentry secrets).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NJKAQ6HAikWGQK7PGZ5Y4E

.github/workflows/publish-fdroid.yml

@@ -12,6 +12,7 @@ jobs:
       contents: write
     env:
       EXPO_PUBLIC_SENTRY_DSN: ${{ secrets.EXPO_PUBLIC_SENTRY_DSN }}
+      EXPO_PUBLIC_POSTHOG_KEY: ${{ secrets.EXPO_PUBLIC_POSTHOG_KEY }}
       SENTRY_AUTH_TOKEN: ${{ secrets.SENTRY_AUTH_TOKEN }}
       SENTRY_ORG: ${{ secrets.SENTRY_ORG }}
       SENTRY_PROJECT: ${{ secrets.SENTRY_PROJECT }}