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

src/stores/connections.ts

@@ -4,6 +4,7 @@ import * as Crypto from "expo-crypto"
 import type { ServerConnection, ConnectionType } from "../lib/types"
 import { createClient, type Client, type Project } from "../lib/sdk"
 import { addBreadcrumb } from "../lib/sentry"
+import { AnalyticsEvent, classifyConnectionError, track } from "../lib/analytics"
 import { buildAuth } from "../lib/auth"
 
 const CONNECTIONS_KEY = "opencode_connections"
@@ -243,6 +244,7 @@ export const useConnections = create<ConnectionsState>((set, get) => ({
   },
 
   testConnection: async (connection, password) => {
+    track(AnalyticsEvent.ConnectionAttempted)
     try {
       const client = createClient({
         baseUrl: connection.url,
@@ -251,9 +253,12 @@ export const useConnections = create<ConnectionsState>((set, get) => ({
       })
 
       await client.global.health()
+      track(AnalyticsEvent.ConnectionSucceeded)
       return { ok: true }
     } catch (error) {
-      return { ok: false, error: error instanceof Error ? error.message : String(error) }
+      const message = error instanceof Error ? error.message : String(error)
+      track(AnalyticsEvent.ConnectionFailed, { error_class: classifyConnectionError(message) })
+      return { ok: false, error: message }
     }
   },