M9: kvitt-import Splitwise importer (SPEC §10.8, FR-17)

CLI per §10.8 with --help, --dry-run and a summary; English and Swedish
headers with member columns matched by name; CSV quoting; payments as
settlements with one allocation; trailing totals row used to reconcile
the account balance (mismatch rolls back, exit 4); non-SEK rows abort
unless --fx; committed category table data/splitwise_categories.csv;
idempotent re-runs through import_key; rows validated with the server's
push checks, audited with source import, no pushes; one BEGIN IMMEDIATE
transaction. Tests over synthetic fixtures only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-import/tests/fixtures/mismatch.csv

@@ -0,0 +1,6 @@
+Date,Description,Category,Cost,Currency,Alex Example,Robin Sample
+
+2025-05-01,Fine row,Groceries,10.00,SEK,5.00,-5.00
+
+2025-05-02,Total balance, , ,SEK,7.00,-7.00
+