fix(export): paginate the archive size estimate and explain scope counts (#1635)

The period branch of estimateArchiveSize ran a single unpaginated
document read with one flat IN() over every posted entry id in the
year: past the PostgREST row cap it silently undercounts, and past a
few hundred entry ids the URL itself blows up. Chunk the id filter
(CHILD_FK_CHUNK) and paginate every read with fetchAllRows, mirroring
what writeDocuments already did (the ZIP content was never affected).

The dialog now says per scope which documents are counted: full
history includes unlinked inbox/receipt documents, a single year only
those linked to posted vouchers. Without that line, a company with
many unlinked receipts reads the count gap as a pagination bug.

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

messages/en.json

@@ -1422,6 +1422,7 @@
     "default_payment_terms_label": "Payment terms (days)",
     "default_account_label": "Default account",
     "default_account_placeholder": "e.g. 5410",
+    "default_account_clear": "Clear default account",
     "default_currency_label": "Default currency",
     "notes_label": "Notes",
     "notes_placeholder": "Internal notes about the supplier...",
@@ -7055,6 +7056,8 @@
     "archive_scope_label": "Scope",
     "archive_scope_all": "Full history",
     "archive_scope_period": "Single fiscal year",
+    "archive_scope_all_note": "All accounting records, including documents not yet linked to any voucher (inbox and unmatched receipts).",
+    "archive_scope_period_note": "Documents linked to posted vouchers in the selected year. Unlinked documents are only included in Full history.",
     "archive_include_docs_label": "Include receipts and supporting documents",
     "archive_include_docs_help": "Voucher attachments (receipts, invoices, PDFs) are packed into the ZIP. Turn off for a smaller archive with bookkeeping data only.",
     "archive_calculating_size": "Calculating size…",