feat(bookkeeping): journal-entry form UX — persistent tabs, focus-advance, opt-in balance fill, period guards (#650)

Captures working-tree changes to the journal-entry form:
- Persist form state across tab switches: forceMount the three bookkeeping tabs (page.tsx) + hide inactive forceMounted panels in the Tabs primitive (tabs.tsx; no-op for non-forceMount tabs).
- AccountCombobox: new onCommit callback fires on a definitive account selection (dropdown pick or full 4-digit entry); the form uses it to auto-advance focus to the debit field (mobile + desktop layouts via refs + visibility check).
- Replace the surprising auto-fill-balancing-amount on account select with opt-in double-click on a debit/credit field (handleFillBalance) — the prior behaviour misfired when splitting across lines.
- Keep exactly one trailing blank row (StrictMode-safe idempotent effect).
- Review-step safety warnings: different-month-than-last-voucher and closed/locked-period notices (paired sv/en strings) — guards against posting to the wrong month/period.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

components/ui/tabs.tsx

@@ -44,6 +44,10 @@ const TabsContent = React.forwardRef<
     ref={ref}
     className={cn(
       "mt-2 ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2",
+      // When a consumer passes `forceMount`, Radix keeps inactive panels in the
+      // DOM (present is always true) but does NOT hide them itself — hide them
+      // here. No-op for non-forceMount tabs, which unmount inactive content.
+      "data-[state=inactive]:hidden",
       className
     )}
     {...props}