feat: import transactions from a CSV bank statement

Fourth of the four ideas picked from the Maybe/Sure feature audit, and the
last one queued up. A 3-step wizard (account -> column mapping -> preview)
at /transactions/import, parsed entirely client-side (new lib/csv.ts —
delimiter auto-detect, 5 date formats, thousands/decimal-separator-aware
amounts) and submitted through a new all-or-nothing POST /transactions/bulk
endpoint, mirroring backup restore's validate-everything-before-touching-
anything approach.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "aurum-frontend",
   "private": true,
-  "version": "0.25.0",
+  "version": "0.26.0",
   "type": "module",
   "scripts": {
     "dev": "vite",