fix(import): migration preview and theater read all fiscal years, not just the newest (#1614)

The /sie-data route parsed only the newest fiscal year's SIE file for the
import preview and the returned SIEData.parsed. Mid-year provider exports
have few or zero vouchers in the newest year, so the first real Fortnox
migration (3 fiscal years, 4153 vouchers) previewed "0 verifikationer"
and drew an almost-empty migration theater while the import itself
landed all 4153 vouchers from the older files.

- New mergeParsedSIEFiles (lib/import/sie-merge.ts): pure, browser-clean
  whole-dataset merge (accounts union first-wins, vouchers concatenated,
  fiscal years union oldest-first re-indexed newest=0, balances and
  issues concatenated, dimensions deduped), with unit tests.
- /sie-data parses each file exactly once, builds the preview from the
  merged parse and returns parsed: merged; response shape unchanged.
  Validation stays newest-file-only so no previously accepted dataset
  is newly rejected.
- /preview drops latestOnly and computes sieStats from the merged parse:
  the connect step's "Hittade X konton och Y verifikationer" line
  renders from THESE stats, so this is where the founder-visible count
  was lying.
- The migration theater spreads its account waves across ~10s and
  births an additional wave on each real step label during the SIE
  phase (progress <= 55), through a shared rate-limited gate, so the
  canvas keeps performing over a multi-minute run. Narration labels and
  progress remain the wizard's real values; reduced motion unchanged.

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

DECISIONS.md

@@ -984,3 +984,5 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-13] Radius ladder locked (convention 16): 4 tiers by role (pill toolbar controls / rounded-xl overlays / rounded-lg surfaces / rounded-sm leaves); rounded-md, bare rounded, rounded-2xl and rounded-[Npx] retired app-wide, hard-failed by check:guards off-ladder-radius. Before: 7 radii in circulation with no rule; one toolbar row on /transactions mixed 4 shape languages.
 [2026-08-13] Toolbar shape language: pills won over "inputs stay rectangles". Search in a page toolbar is a pill (ToolbarSearch, h-8) matching chips/pickers/buttons beside it; the same search inside a dialog or form keeps rounded-lg Input. Rationale: convention 8 already made pickers pill chips and convention 3 made buttons pills, so the rectangle search/segmented were the odd ones out; one row = one shape reads as trust.
 [2026-08-13] Dialogs promoted 8px -> 12px (rounded-xl): SettingsModal, slide-over and CommandPalette were already 12px, so dialog.tsx was the overlay-tier outlier, not the rule.
+[2026-08-14] Migration /preview fetches ALL allowed fiscal years (dropped latestOnly): the connect step's "Hittade X konton och Y verifikationer" renders from /preview's sieStats, not /sie-data's generateImportPreview, so fixing only /sie-data would have left the founder-reported "0 verifikationer" (actual: 4153) on screen. Costs one SIE export per extra year at connect time, the same work /sie-data repeats right after; honest counts won over latency.
+[2026-08-14] /sie-data validation stays newest-file-only (not per-file, not on the merged parse): preserves exactly which datasets are accepted today, and validateSIEFile assumes single-file invariants (balance yearIndexes relative to ONE current year) that mergeParsedSIEFiles deliberately does not preserve. Older files' problems still surface per-file at import time.