feat(mobile): ping last_active_at on app foreground

[dylanjeffers]

Summary

• Adds pingActivity function to AudiusBackend — fire-and-forget POST to /v1/users/me/ping with signature auth
• Adds useActivityPing hook that fires on every foreground transition when user is logged in
• Wires the hook into RootScreen alongside useResetNotificationBadgeCount

Related PRs

• api: feat: add last_active_at column and /v1/users/me/ping endpoint
• pedalboard: notifications: query users.last_active_at for inactive user detection

Test plan

• Open app from background, verify network request to /v1/users/me/ping fires
• Verify no request fires when logged out
• Verify app behavior is unaffected if ping fails (fire-and-forget)
• TypeScript compiles cleanly (npx tsc --noEmit --project packages/mobile/tsconfig.json)

🤖 Generated with Claude Code

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