feat(onboarding): activation quick wins on the Hem checklist (#1461)

* feat(onboarding): activation quick wins on the Hem checklist

First implementation slice of the approved activation concept
(artifact de543d57, dev_docs/onboarding_activation_analysis.md §9):

- New "Kvitton och underlag" checklist step, gated on the invoice-inbox
  extension like the Skatteverket step; done once the company has ever
  received an inbox item (email/WhatsApp/upload). Non-AI companies route
  to billing, matching the assistant step.
- Personalized VAT line in the Skatteverket step: the company's real next
  momsdeklaration due date from the deadlines table, with an explicit
  "välj momsperiod" prompt when vat_registered is set but moms_period is
  null (that state silently generates zero VAT deadlines).
- Truthful Att göra empty state: while the setup checklist is open and no
  journal entry is posted, the all-clear reads "Bokföringen är tom än"
  instead of a false "Allt klart!".
- Activation funnel events (onboarding_setup_step_started / _completed /
  _dismissed) via posthog-js, mirroring the existing guarded capture
  pattern; sandbox never renders the block so no extra gate is needed.

Pure helpers live in lib/onboarding/checklist.ts with tests; step
numbering now adapts to both optional extensions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs: record the receipts-signal and moms-period-guard decisions

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(onboarding): review triage: error-safe emptyLedger, stale-state guard, copy

- A failed posted-entries count no longer reads as an empty ledger.
- Confirming a suggested match books an entry, so the empty-ledger copy
  retires for the rest of the session (postedSinceLoad).
- 'Bokföringen är tom än så länge' reads naturally.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

types/index.ts

@@ -2354,6 +2354,8 @@ export interface OnboardingProgress {
   hasSIEImport: boolean
   /** True when the active user has a stored Skatteverket OAuth token. */
   hasSkatteverketConnected: boolean
+  /** True when the company has ever received an item in the document inbox. */
+  hasInboxItems: boolean
 }
 
 export type InitialSetupPath = 'migration' | 'bank' | 'fresh'