feat(import): folder pick for underlag attach (#2189) (#2230)

A migration's underlag arrives as one folder, and Ctrl+A in the file
picker is not obvious to everyone. The underlag wizard gets a second,
outlined "Välj mapp" button next to "Välj filer", backed by a hidden
webkitdirectory input.

A directory pick ignores the accept list and returns every file in the
tree, so the handler keeps only the document types the attach route takes
(PDF, JPEG, PNG, WebP), drops dotfiles, and explains itself when nothing
qualifies. The plan keys on file.name, so nested folders flatten harmlessly.

Closes #2189


Claude-Session: https://claude.ai/code/session_01QPQLwHNEiQfiCNLSMzXMiQ

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

messages/en.json

@@ -7451,6 +7451,8 @@
     "underlag_intro": "A SIE file carries the bookkeeping but not the receipts. If your old system named each receipt after its verifikat, they can be attached automatically, with no AI reading required.",
     "underlag_intro_formats": "Filenames that work: A31_abc123.pdf, A31.pdf, A-31 receipt.pdf, 2024-A-31.pdf. Matching uses the voucher number from your old system, not the number here.",
     "underlag_pick_files": "Pick files",
+    "underlag_pick_folder": "Pick a folder",
+    "underlag_folder_no_documents": "The folder holds no PDF or image files (PDF, JPEG, PNG, WebP).",
     "underlag_year_help": "The old system restarts voucher numbering every year, so A31 only identifies a verifikat within one year. Pick the year the export came from. Files are never attached to any other year.",
     "underlag_year_required": "Pick a fiscal year above to continue. If the year you want is missing, its bookkeeping was not imported via SIE.",
     "underlag_year_label": "Which fiscal year are these receipts from?",

messages/sv.json

@@ -7451,6 +7451,8 @@
     "underlag_intro": "En SIE-fil innehåller bokföringen men inte underlagen. Om ditt gamla system namngav varje underlag efter sitt verifikat kan de kopplas automatiskt, utan AI-tolkning.",
     "underlag_intro_formats": "Filnamn som fungerar: A31_abc123.pdf, A31.pdf, A-31 kvitto.pdf, 2024-A-31.pdf. Matchningen sker mot verifikatnumret i ditt gamla system, inte mot numret här.",
     "underlag_pick_files": "Välj filer",
+    "underlag_pick_folder": "Välj mapp",
+    "underlag_folder_no_documents": "Mappen innehåller inga PDF- eller bildfiler (PDF, JPEG, PNG, WebP).",
     "underlag_year_help": "Verifikatnumret i filnamnet räknas om varje år i det gamla systemet, så A31 pekar bara ut ett verifikat inom ett år. Välj det år exporten kommer från. Filerna kopplas aldrig till något annat år.",
     "underlag_year_required": "Välj ett räkenskapsår ovan för att fortsätta. Saknas det år du söker har bokföringen för det året inte importerats via SIE.",
     "underlag_year_label": "Vilket räkenskapsår gäller underlagen?",